Published in Journals                  Unpublished (in arXiv) 

A modular point contact spectroscopy probe for sub-Kelvin applications

Shekhar Das and Goutam Sheet

Rev. Sci. Instrum. 90, 103903 (2019)

We present the construction and performance of a plug-n-play type point contact spectroscopy probe equipped with a piezodriven coarse approach mechanism for sub-Kelvin applications. A modular assembly has been built, which can be placed in or taken out from a sub-Kelvin socket mounted inside a He3 cryostat (with a 7 T magnet) using an external manipulator. A simple transfer process using a removable vertical manipulator combined with a load-lock chamber makes the exchange of the sample/tip rig very easy without breaking the vacuum insulation of the sample chamber. We have successfully performed point-contact spectroscopic studies on certain elemental superconductors with low Tc. We have also used the probe to perform spectroscopic measurements on another low temperature superconductor PdTe2, which is a Dirac semimetal in the normal state. We present such data to demonstrate the functionality of the probe. The probe can also be made adaptable for other transport measurements such as Hall effect, four probe resistivity, scanning tunneling spectroscopy, etc.

Generation of strain-induced pseudo-magnetic field in a doped type-II Weyl semimetal

Suman Kamboj, Partha Sarathi Rana, Anshu Sirohi, Aastha Vasdev, Manasi Mandal, Sourav Marik, Ravi Prakash Singh, Tanmoy Das and Goutam Sheet

PHYSICAL REVIEW B 100, 115105 (2019)

In Weyl semimetals, there is an intriguing possibility of realizing a pseudo-magnetic field in the presence of small strain due to certain special cases of static deformations. This pseudo-magnetic field can be large enough to form quantized Landau levels and thus become observable in Weyl semimetals. In this paper we experimentally show the emergence of a pseudo-magnetic field (∼3 T) by scanning tunneling spectroscopy on the doped Weyl semimetal Re-MoTe2, where distinct Landau-level oscillations in the tunneling conductance are clearly resolved. The crystal lattice is intrinsically strained where large area scanning tunneling microscopy imaging of the surface reveals differently strained domains where atomic scale deformations exist forming topographic ripples with varying periodicity in the real space. The effect of the pseudo-magnetic field is clearly resolved in areas under maximum strain.

Tip-induced Superconductivity Coexisting with Preserved Topological Properties in Line-nodal Semimetal ZrSiS

Leena Aggarwal, Chandan K. Singh, Mohammad Aslam, Ratnadwip Singha, Arnab Pariari, Sirshendu Gayen, Mukul Kabir, Prabhat Mandal, and Goutam Sheet

Accepted in Journal Of Physics-Condensed Matter Physics(2019)

ZrSiS was recently shown to be a new material with topologically non-trivial band structure which exhibits multiple Dirac nodes and a robust linear band dispersion up to an unusually high energy of 2 eV. Such a robust linear dispersion makes the topological properties of ZrSiS insensitive to perturbations like carrier doping or lattice distortion. Here we show that a novel superconducting phase with a remarkably high Tc of 7.5 K can be induced in single crystals of ZrSiS by a non-superconducting metallic tip of Ag. From first-principles calculations we show that the observed superconducting phase might originate from dramatic enhancement of density of states due to the presence of a metallic tip on ZrSiS. Our calculations also show that the emerging tip-induced superconducting phase co-exists with the well preserved topological properties of ZrSiS.

Ultrathin Free-standing Nanosheets of Bi2O2Se: Room Temperature Ferroelectricity in Self-assembled Charged Layered Heterostructure

Tanmoy Ghosh, Manisha Samanta, Aastha Vasdev, Kapildeb Dolui, Jay Ghatak, Tanmoy Das,Goutam Sheet* and Kanishka Biswas*

Accepted in Nanoletters (2019)

Ultrathin ferroelectric semiconductors with high charge carrier mobility are much coveted systems for the advancement of various electronic and optoelectronic devices. However, in traditional oxide ferroelectric insulators, the ferroelectric transition temperature decreases drastically with decreasing material thickness and ceases to exist below certain critical thickness owing to depolarizing field. Herein, we show the emergence of an ordered ferroelectric ground state in ultrathin (~2 nm) single crystalline nanosheet of Bi2O2Se at room temperature. Free-standing ferroelectric nanosheets, in which oppositely charged alternating layers are self-assembled together by electrostatic interactions, are synthesized by a simple, rapid and scalable wet chemical procedure at room temperature. The existence of ferroelectricity in Bi2O2Se nanosheets is confirmed by dielectric measurements and piezoresponse force spectroscopy. The spontaneous orthorhombic distortion in the ultrathin nanosheets breaks the local inversion symmetry, thereby results in ferroelectricity. The local structural distortion and the formation of spontaneous dipole moment were directly probed by atomic resolution scanning transmission electron microscopy and density functional theory calculations.

Understanding Basic Concepts of Topological Insulators Through Su-Schrieffer-Heeger (SSH) Model

Navketan Batra and Goutam Sheet

Accepted in Resonance: Journal of Science Education (2019)

Topological insulators are a new class of materials that have attracted significant attention in contemporary condensed matter physics. They are different from the regular insulators and they display novel quantum properties that also involve the idea of ‘topology’, an area of mathematics. Some of the fundamental ideas behind the topological insulators, particularly in low-dimensional condensed matter systems such as poly-acetylene chains, can be understood using a simple one-dimensional toy model popularly known as the Su-Schrieffer-Heeger model or the SSH model. This model can also be used as an introduction to the topological insulators of higher dimensions. Here we give a concise description of the SSH model along with a brief review of the background physics and attempt to understand the ideas of topological invariants, edge states, and bulk-boundary correspondence using the model.

Temperature dependent transport spin-polarization in the low Curie temperature complex itinerant ferromagnet EuTi1−x Nbx O3

Suman Kamboj, Deepak K Roy, Susmita Roy, Rajeswari Roy Chowdhury, Prabhat Mandal, Mukul Kabir, and Goutam Sheet

J. Phys.: Condens. Matter 31 (2019) 415601

The physical systems with ferromagnetism and ‘bad’ metallicity hosting unusual transport properties are playgrounds of novel quantum phenomena. Recently EuTi1−xNbxO3 emerged as a ferromagnetic system where non-trivial temperature dependent transport properties are observed due to coexistence and competition of various magnetic and non-magnetic scattering processes. In the ferromagnetic state, the resistivity shows a T2 temperature dependence possibly due to electron–magnon scattering and above the Curie temperature Tc, the dependence changes to T3/2 behavior indicating a correlation between transport and magnetic properties. In this paper, we show that the transport spin-polarization (Pt) in EuTi1−xNbxO3, a low Curie temperature ferromagnet, is as high (∼40%) as that in some of the metallic ferromagnets with high Curie temperatures. In addition, owing to the low Curie temperature of EuTi1−xNbxO3, the temperature (T) dependence of Pt could be measured systematically up to Tc which revealed a proportionate relationship with magnetization Ms versus T. This indicates that such proportionality is far more universally valid than the ferromagnets with ideal parabolic bands. Furthermore, our band structure calculations not only helped to understand the origin of such high spin polarization in EuTi1−xNbxO3 but also provided a route to estimate the Hubbard U parameter in complex metallic ferromagnets, in general, using experimental inputs.

Realization of Diverse Waveform Converter from a Single Nano-scale Lateral p-n Junction Cu2S-CdS Heterostructure

Amit Dalui, Mrityunjay Pandey, Piyush Kanti Sarkar, Bapi Pradhan, Aastha Vasdev, Nabin Baran Manik, Goutam Sheet, and Somobrata Acharya

Accepted in Applied Materials & Interfaces (2019)

A differentiator is an electronic component used to accomplish the mathematical operation of calculus functions of differentiation for shaping different waveforms. Differentiators are used in numerous areas of electronics including electronic analog computers, wave shaping circuits and frequency modulators. Conventional differentiators are fabricated using active operational amplifier or using passive resistor-capacitor combination. Here we report that a single Cu2SCdS heterostructure acts a differentiator for performing numerical functions of input waveforms conversion into different shapes. When a rectangular wave signal is applied through tip of conductive atomic force microscope, a spike-like wave signal is obtained from the Cu2SCdS heterostructure. The Cu2SCdS differentiator is able to convert sine wave signal into a cosine wave signal and a triangular wave signal into a square wave signal similar to the classical differentiators. The finding of a nano-scale differentiator at extremely small length scales may have profound applications in different domains of electronics.

Multiband superconductivity in Mo8Ga41 driven by a site-selective mechanism

Anshu Sirohi, Surabhi Saha, Prakriti Neha, Shekhar Das, Satyabrata Patnaik, Tanmoy Das, Goutam Sheet

PHYSICAL REVIEW B, Volume 99,  Article 054503 (2019)

The family of the endohedral gallide cluster compounds recently emerged as a new family of superconductors which is expected to host systems displaying unconventional physics. Mo8Ga41 is an important member of this family which shows relatively large Tc ∼ 10 K and has shown indications of strong electron-phonon coupling and multiband superconductivity. Here, through direct measurement of superconducting energy gap by scanning tunneling spectroscopy (STS), we demonstrate the existence of two distinct superconducting gaps of magnitude 0.85 and 1.6 meV, respectively, in Mo8Ga41. Both gaps are seen to be conventional in nature as they evolve systematically with temperature as per the predictions of BCS theory. Our band structure calculations reveal that only two specific Mo sites in a unit cell contribute to superconductivity where only dxz/dyz and dx2−y2 orbitals have strong contributions. Our analysis indicates that the site-elective contribution governs the two-gap nature of superconductivity in Mo8Ga41.

Mixed type I and type II superconductivity due to intrinsic electronic inhomogeneities in the type II Dirac semimetal PdTe2

Anshu Sirohi, Shekhar Das, Priyo Adhikary, Rajeswari Roy Chowdhury, Amit Vashist, Yogesh Singh, Sirshendu Gayen, Tanmoy Das and Goutam Sheet

Journal of physics: Condensed matter, Volume 31, Article 085701 (2019)

The type II Dirac semimetal PdTe is unique in the family of topological parent materials because it displays a superconducting ground state below 1.7 K. Despite wide speculation on the possibility of an unconventional topological superconducting phase, tunneling and heat capacity measurements revealed that the superconducting phase of PdTe follows predictions of the microscopic theory of Bardeen, Cooper and Schrieffer for conventional superconductors. The superconducting phase in PdTe is further interesting because it also displays properties that are characteristic of type-I superconductors and are generally unexpected for binary compounds. Here, from scanning tunneling spectroscopic measurements we show that the surface of PdTe displays intrinsic electronic inhomogeneities in the normal state which leads to a mixed type I and type II superconducting behaviour along with a spatial distribution of critical fields in the superconducting state. Understanding of the origin of such inhomogeneities may be important for understanding the topological properties of PdTe in the normal state.

Mesoscopic Superconductivity above 10K in Silicon Point contacts.

Anshu Sirohi, Sirshendu Gayen, Mohammad Aslam, Goutam Sheet

Applied Physics Letters, Volume 113, 10 242601 (2018)

Silicon, perhaps the most ubiquitously used material in the digital age of today, has also been a material of choice for testing the fundamental differences between various electronic ground states, e.g., metals and insulators. This is mainly because ultimate control has been achieved in growing extremely pure silicon crystals and doping them with varying concentrations of charge carriers and their mobility. Here, we show that by forming mesoscopic point contacts with non-superconducting metals on insulating (doped) silicon, it is possible to obtain a superconducting phase with a remarkably high critical temperature above 10 K and an average superconducting energy gap of 2 meV. Apart from its importance in advancing the understanding of nanoscale superconductivity, this discovery is also expected to boost the efforts to realize silicon based superconducting devices with far reaching application potential.

Low-energy excitations and non-BCS superconductivity in Nbx-Bi2Se3

Anshu Sirohi, Shekhar Das, Prakriti Neha, Karn S. Jat, Satyabrata Patnaik, and Goutam Sheet

PHYSICAL REVIEW B, Volume 98, 27 September 2018, Article 094523 (2018)

When certain elemental metals like Cu, Sr and Nb are intercalated between the layers of Bi2Se3, a topological insulator, the intercalated systems superconduct with critical temperatures around 3 K. Naturally, in all these cases, the possibility of topological superconductivity was suggested and explored. However, in cases of Cu and Sr intercalated systems, the low-temperature scanning tunneling microscopy (STM) experiments revealed fully formed gaps where no signature of low-energy states, a requisite for topological superconductivity, was found. Here, through STM spectroscopy down to 400 mK we show that in Nbx-Bi2Se3 (x = 0.25), the spectra deviate from a BCS-like behavior and the spectral weight at low-bias is large. Our observations are consistent with the idea that the order parameter of Nbx-Bi2Se3 is nodal. Therefore, our results conclude that compared to other members of the family, Nbx-Bi2Se3 has a stronger possibility of being a topological superconductor.

Shekhar Das, Anshu Sirohi, Gaurav Kumar Gupta, Suman Kamboj, Aastha Vasdev, Sirshendu Gayen, Prasenjit Guptasarma, Tanmoy Das, and Goutam Sheet

PHYSICAL REVIEW B 97, 235306 (2018)

Majority of the A2B3-type chalcogenide systems with strong spin-orbit coupling (SOC), such as Bi2Se3, Bi2Te3, and Sb2Te3, etc., are topological insulators. One important exception is Sb2Se3 where a topological nontrivial phase was argued to be possible under ambient conditions, but such a phase could be detected to exist only under pressure. In this paper, we show that Sb2Se3 like Bi2Se3 displays a generation of highly spin-polarized current under mesoscopic superconducting point contacts as measured by point-contact Andreev reflection spectroscopy. In addition, we observe a large negative and anisotropic magnetoresistance of the mesoscopic metallic point contacts formed on Sb2Se3. Our band-structure calculations confirm the trivial nature of Sb2Se3 crystals and reveal two trivial surface states one of which shows large spin splitting due to Rashba-type SOC. The observed high spin polarization and related phenomena in Sb2Se3 can be attributed to this spin splitting.

Dynamic Surface Modification due to Effusion of Na in Na2IrO3

Aastha Vasdev, Lalit Yadav, Suman Kamboj, Kavita Mehlawat, Yogesh Singh, and Goutam Sheet

Journal of Applied Physics 124, 055102 (2018)

The honeycomb lattice iridate Na2IrO3 shows frustrated magnetism and can potentially display Kitaev-like exchange interactions. Recently, it was shown that the electronic properties of the surface of crystalline Na2IrO3 can be tuned by Ar plasma treatment in a controlled manner leading to various phases of matter ranging from a fully gapped to a metallic surface, where the possibility of a charge-density wave (CDW) like transition is also expected. Here, through direct imaging with an atomic force microscope (AFM) in air, we show that the surface of crystalline Na2IrO3 evolves rapidly as elemental Na effuses out of the interleave planes to the surface and undergoes sublimation thereby disappearing from the surface gradually over time. Using conductive AFM we recorded a series of topographs and surface current maps simultaneously and found that the modification of the surface leads to change in the electronic properties in a dynamic fashion until the whole system reaches a dynamic equilibrium. These observations are important in the context of the exotic electronic and magnetic properties that the surface of Na2IrO3 displays.

Shekhar Das, Amit, Anshu Sirohi, Lalit Yadav, Sirshendu Gayen, Yogesh Singh, and Goutam Sheet

PHYSICAL REVIEW B 97, 014523 (2018)

The transition metal dichalcogenide PdTe2 was recently shown to be a unique system where a type-II Dirac semimetallic phase and a superconducting phase coexist. This observation has led to wide speculation on the possibility of the emergence of an unconventional topological superconducting phase in PdTe2. Here, through direct measurement of the superconducting energy gap by scanning tunneling spectroscopy, and temperature and magnetic-field evolution of same, we show that the superconducting phase in PdTe2 is conventional in nature. The superconducting energy gap is measured to be 326 μeV at 0.38 K, and it follows a temperature dependence that is well described within the framework of Bardeen-Cooper-Schrieffer’s theory of conventional superconductivity. This is surprising because our quantum oscillation measurements confirm that at least one of the bands participating in transport has topologically nontrivial character.

Large enhancement of superconductivity in Zr point contacts

Mohammad Aslam, Chandan K. Singh, Shekhar Das, Ritesh Kumar, Soumya Datta, Soumyadip Haldar, Sirshendu gayen, Mukul Kabir and Goutam Sheet

J Phys Condens Matter, 30, 255002 (2018)

For certain complex superconducting systems, the superconducting properties get enhanced under mesoscopic point contacts made of elemental non-superconducting metals. However, understanding of the mechanism through which such contact induced local enhancement of superconductivity happens has been limited due to the complex nature of such compounds. In this paper we present a large enhancement of superconducting transition temperature T c and superconducting energy gap Δ in a simple elemental superconductor Zr. While bulk Zr shows a critical temperature around 0.6 K, superconductivity survives at Ag/Zr and Pt/Zr point contacts up to 3 K with a corresponding five-fold enhancement of Δ. Further, the first-principles calculations on a model system provide useful insights. We show that the enhancement in superconducting properties can be attributed to a modification in the electron-phonon coupling accompanied by an enhancement of the density of states which involves the appearance of a new electron band at the Ag/Zr interfaces.

Modulating Capacitive Response of MoS2 Flake by Controlled Nanostructuring through Focused Laser Irradiation

Renu Rani, Anirban Kundu, Mohammad Balal, Goutam Sheet, Kiran Shankar Hazra

Nanotechnology IOP PublishingVolume 29Number 34 (2018)

Unlike graphene nanostructures, various physical properties of nanostructured MoS2 have remained unexplored due to the lack of established fabrication routes. Herein, we have reported unique electrostatic properties of MoS2 nanostructures, fabricated in a controlled manner of different geometries on 2D flake by using focused laser irradiation technique. Electrostatic force microscopy has been carried out on MoS2 nanostructures by varying tip bias voltage and lift height. The analysis depicts no contrast flip in phase image of the patterned nanostructure due to the absence of free surface charges. However, prominent change in phase shift at the patterned area is observed. Such contrast changes signify the capacitive interaction between tip and nanostructures at varying tip bias voltage and lift height irrespective of their shape and size. 

Suman Kamboj , Shekhar Das , Anshu Sirohi , Rajeswari Roy Chowdhury ,  Sirshendu Gayen , Vishal K. Maurya , Satyabrata Patnaik , and Goutam Sheet
Journal of Physics: Condensed MatterVolume 30Number 35 (2018)

The surface states of topological insulators (TI) are protected by time reversal symmetry and they display intrinsic spin helicity where the momentum of the charge carriers decides their spin states. As a consequence, a current injected through the surface states becomes spin polarized and this transport spin-polarization leads to a proportionate suppression of Andreev reflection in superconductor/TI junctions. Here we show that upon doping Bi2Se3 with Mn, the transport spin-polarization is seen to be monotonically suppressed. The parent compound Bi2Se3 is found to exhibit a transport spin-polarization of about 63% whereas crystals with 10% Mn doping show transport spin-polarization of about 48%. This suppression is accompanied by an increasing ferromagnetic order of the crystals with Mn doping. Scanning tunneling spectroscopy shows that the topological protection of the surface states reduce due to Mn doping. The net measured transport spin-polarization is due to a competition of this effect with the increased magnetization on Mn doping. The present results provide important insights for the choice of magnetic topological insulators for spintronic applications.

Mohammad Balal, Shilpa Sanwlani, Neha Wadehra, Suvankar Chakraverty, Goutam Sheet

Applied Physics Letters 110, 261604 (2017)

The high-mobility two-dimensional electron gases (2DEG) formed at the interfaces between certain insulating perovskite oxides have known to be a playground of exotic physical orders like superconductivity and ferromagnetism and their inter-coupling. There have been efforts to accomplish electronic confinement at such interfaces of oxide heterostructures through nano-structuring of the surface. In this paper, we report writing and erasing charge domains on such an oxide heterostructure LaVO3/SrTiO3 using a conductive AFM cantilever. We have patterned these domains in a periodic fashion in order to create artificial lattices on the surface. Through kelvin probe microscopy, electrostatic force microscopy, and conductivity mapping of such artificial lattices, we found that the domains not only trap charge carriers but also develop a controllable potential landscape on the surface which coincides with a modulation of local electrical conductivity. The ability to pattern such nanostructures reversibly offers unprecedented opportunities of realizing ultra-high storage density devices in high mobility oxide heterostructures.

Mohammad Aslam, Sirshendu Gayen, Avtar Singh, Masashi Tanaka, Takuma Yamaki, Yoshihiko Takano, Goutam Sheet

Solid State Communications 264, 26 (2017)

From field-angle dependent Andreev reflection spectroscopy on single crystals of La(O,F)BiSeS, which belongs to the recently discovered BiCh2 (Ch = S, Se) based layered superconductors, we found that the superconductivity in La(O,F)BiSeS is highly anisotropic. We measured a superconducting energy gap of 0.61 meV for current injected along c-axis at 1.5 K. Detailed temperature and magnetic field dependent studies of the gap also reveal the presence of unconventional pairing in La(O,F)BiSeS. We show that the observed anisotropic superconducting properties can be attributed to the anisotropy in the superconducting order parameter with a complex symmetry in superconducting La(O,F)BiSeS.

Mesoscopic superconductivity and high spin polarization coexisting at metallic point contacts on the Weyl semimetal TaAs

Leena Aggarwal, Sirshendu Gayen, Shekhar Das, Ritesh Kumar, Vicky Süß, Claudia Felser, Chandra Shekhar
& Goutam Sheet

Nature Communications 8, 13974 (2017)

A Weyl semimetal is a topologically non-trivial phase of matter that hosts mass-less Weyl
fermions, the particles that remained elusive for more than 80 years since their theoretical
discovery. The Weyl semimetals exhibit unique transport properties and remarkably high
surface spin polarization. Here we show that a mesoscopic superconducting phase with
critical temperature Tc=7 K can be realized by forming metallic point contacts with silver
(Ag) on single crystals of TaAs, while neither Ag nor TaAs are superconductors. Andreev
reflection spectroscopy of such point contacts reveals a superconducting gap of 1.2 meV that
coexists with a high transport spin polarization of 60% indicating a highly spin-polarized
supercurrent flowing through the point contacts on TaAs. Therefore, apart from the discovery
of a novel mesoscopic superconducting phase, our results also show that the point contacts
on Weyl semimetals are potentially important for applications in spintronics.

Unconventional superconductivity at mesoscopic point contacts on the 3D Dirac semimetal Cd3As2

Leena Aggarwal, Abhishek Gaurav, Gohil S. Thakur, Zeba Haque, Ashok K. Ganguli & Goutam Sheet

Nature Materials 15 32-37 (2016).

Three-dimensional (3D) Dirac semimetals exist close to topological phase boundaries which, in principle, should make it possible to drive them into exotic new phases, such as topological superconductivity, by breaking certain symmetries. A practical realization of this idea has, however, hitherto been lacking. Here we show that the mesoscopic point contacts between pure silver (Ag) and the 3D Dirac semimetal Cd3As2 exhibit unconventional superconductivity with a critical temperature (onset) greater than 6 K whereas neither Cd3As2 nor Ag are superconductors. A gap amplitude of 6.5 meV is measured spectroscopically in this phase that varies weakly with temperature and survives up to a remarkably high temperature of 13 K, indicating the presence of a robust normal-state pseudogap. The observations indicate the emergence of a new unconventional superconducting phase that exists in a quantum mechanically confined region under a point contact between a Dirac semimetal and a normal metal.

Unexpected superconductivity at nanoscale junctions made on the topological crystalline insulator Pb0. 6Sn0. 4Te

Shekhar Das, Leena Aggarwal, Subhajit Roychowdhury, Mohammad Aslam, Sirshendu Gayen, Kanishka Biswas, Goutam Sheet

Applied Physics Letters 109 (13), 132601 (2016)

Discovery of exotic phases of matter from the topologically non-trivial systems not only makes the research on topological materials more interesting but also enriches our understanding of the fascinating physics of such materials. Pb0.6Sn0.4Te was recently shown to be a topological crystalline insulator. Here, we show that by forming a mesoscopic point-contact using a normal non-superconducting elemental metal on the surface of Pb0.6Sn0.4Te, a superconducting phase is created locally in a confined region under the point-contact. This happens when the bulk of the sample remains to be non-superconducting, and the superconducting phase emerges as a nano-droplet under the point-contact. The superconducting phase shows a high transition temperature Tc that varies for different point-contacts and falls in a range between 3.7 K and 6.5 K. Therefore, this Letter presents the discovery of a superconducting phase on the surface of a topological crystalline insulator, and the discovery is expected to shed light on the mechanism of induced superconductivity in topologically non-trivial systems in general.

Enhanced zero-bias conductance peak and splitting at mesoscopic interfaces between an s-wave superconductor and a 3D Dirac semimetal

Leena Aggarwal, Sirshendu Gayen, Shekhar Das, Gohil S. Thakur, Ashok K. Ganguli, Goutam Sheet

Applied Physics Letters 109 (25), 252602 (2016)

Mesoscopic point contacts between elemental metals and the topological 3D Dirac semimetal Cd3As2 have been recently shown to be superconducting with unconventional pairing while Cd3As2 itself does not superconduct. Here we show that the same superconducting phase at mesoscopic interfaces on Cd3As2 can be induced with a known conventional superconductor Nb where a pronounced zero-bias conductance peak is observed which undergoes splitting in energy under certain conditions. The observations are consistent with the theory of the emergence of Andreev bound states due to the presence of a pair potential with broken time reversal symmetry. The data also indicate the possibility of Majorana bound states as expected at the interfaces between s-wave superconductors and topologically non-trivial materials with a high degree of spin-orbit coupling.

High spin polarization and the origin of unique ferromagnetic ground state in CuFeSb

CuFeSb is isostructural to the ferro-pnictide and chalcogenide superconductors and it is one of the fewmaterials in the family that are known to stabilize in a ferromagnetic ground state. Majority of the members of this family are either superconductors or antiferromagnets. Therefore, CuFeSb may be used as an ideal source of spin polarized current in spin-transport devices involving pnictide and the chalcogenidesuperconductors. However, for that the Fermi surface of CuFeSb needs to be sufficiently spin polarized. In this paper we report direct measurement of transport spin polarization in CuFeSb by spin-resolved Andreev reflection spectroscopy. From a number of measurements using multiple superconducting tips we found that the intrinsic transport spin polarization in CuFeSb is high (∼47%). In order to understand the unique ground state of CuFeSb and the origin of large spin polarization at the Fermi level, we have evaluated the spin-polarized band structure of CuFeSb through first principles calculations. Apart from supporting the observed 47% transport spin polarization, such calculations also indicate that the Sb-Fe-Sb angles and the height of Sb from the Fe plane are strikingly different for CuFeSb than the equivalent parameters in other members of the same family thereby explaining the origin of the unique ground state of CuFeSb.

Transport spectroscopy on trapped superconducting nano-islands of Pb: signature of unconventional pairing

Anshu Sirohi · Preetha Saha · Sirshendu Gayen · Avtar Singh · Goutam Sheet

Nanotechnology 27 285701 (2016).

Elemental bulk lead (Pb) is a conventional type I, spin-singlet (s-wave) superconductor with a critical temperature Tc = 7.2 K and a critical magnetic field Hc = 800 Oe. However, it is known that at mesoscopic length scales, like in point-contact geometries, Pb shows significantly higher critical field, sometimes up to several Tesla. We have used this property to trap a small superconducting nano-droplet of Pb by forming a metallic point contact on Pb and then applying a magnetic field larger than 800 Oe that drives the bulk of the material non-superconducting. From systematic magnetic field dependent behaviour of the point-contact spectra measured across such a trapped island of Pb we show that the superconducting order parameter of mesoscopic Pb mixes non-trivially with magnetic field possibly due to the emergence of a local spin-triplet component at such length scales. From comparative studies with Nb-based point contacts we surmise that the strong spin-orbit coupling in Pb leads to the emergence of the unconventional component in the order parameter of mesoscopic Pb.

Evidence of a pseudogap driven by competing orders of multi-band origin in the ferromagnetic superconductor Sr0. 5Ce0. 5FBiS2

Mohammad Aslam, Arpita Paul, Gohil S. Thakur, Sirshendu Gayen, Ritesh Kumar, Avtar Singh, Shekhar Das, Umesh Waghmare, Ashok K. Ganguli, Goutam Sheet

Journal of Physics: Condensed Matter, 28 19 (2016).

From temperature and magnetic field dependent point-contact spectroscopy on the ferromagnetic

superconductor Sr0.5Ce0.5FBiS2 (bulk superconducting Tc= 2.5 K) we observe (a) a pseudogap in

the normal state that sustains to a remarkably high temperature of 40 K and (b) two-fold enhance-

ment of Tcupto 5 K in the point-contact geometry. In addition, Andreev reflection spectroscopy

reveals a superconducting gap of 6 meV for certain point-contacts suggesting that the mean field Tc

of this system could be approximately 40 K, the onset temperature of pseudo-gap. Our results sug-

gest that quantum fluctuations originating from other competing orders in Sr0.5Ce0.5FBiS2forbid a

global phase coherence at high temperatures thereby suppressing Tc. Apart from the known order-

ing to a ferromagnetic state, our first-principles calculations reveal nesting of a multi-band Fermi

surface and a significant electron-phonon coupling that could result in charge density wave-like

instabilities.

The role of substrates and environment in piezoresponse force microscopy: A case study with regular glass slides

Shilpa Sanwlani, Mohammad Balal, Shubhra Jyotsna, Goutam Sheet

Solid State Communications 246, 17-22 (2016)

Piezoresponse force microscopy (PFM) is a powerful tool for probing nanometer-scale ferroelectric and piezoelectric properties. Hysteretic switching of the phase and amplitude of the PFM response are believed to be the hallmark of ferroelectric and piezoelectric behavior respectively. However, the application of PFM is limited by the fact that similar hysteretic effects may also arise from mechanisms not related to ferroelectricity or piezoelectricity. In this paper we report our studies on regular glass slides that show ferroelectric-like signal without being ferroelectric and frequently used as a substrate in PFM experiments. We demonstrate how the substrates and other environmental factors like relative humidity and experimental conditions may influence the PFM results on novel materials.

Local ferroelectricity in thermoelectric SnTe above room temperature driven by competing phonon instabilities and soft resonant bonding

Leena Aggarwal, Ananya Banik, Shashwat Anand, Umesh V. Waghmare, Kanishka Biswas, Goutam Sheet

Journal of Materiomics (2016).

We report direct observation of local ferroelectric ordering above room temperature in rocksalt SnTe, which is a topological crystalline insulator and a good thermoelectric material. Although SnTe is known to stabilize in a ferroelectric ground state (rhombohedral phase) below ~100 K, at high temperatures it was not expected to show any ferroelectric ordering forbidden by its globally centro-symmetric crystal structure (Fm-3m). Here, we show that SnTe exhibits local ferroelectric ordering that is robust above room temperature through direct imaging of ferroelectric domains by piezoresponse force microscopy and measurement of local polarization switching using switching spectroscopy. Using first-principles theoretical analysis, we show how the local ferroelectricity arises from soft bonding and competing phonon instabilities at intermediate wavelengths, which induce local Sn-off centering in the otherwise cetrosymmetric SnTe crystal structure. The results make SnTe an important member of the family of new multi-functional materials namely the ferroelectric-thermoelectrics.

A series of s-block (Ca, Sr and Ba) metal–organic frameworks: synthesis and structure–property correlation

K. S. Asha, Madhuri Makkitaya, Anshu Sirohi, Lalit Yadav, Goutam Sheet and Sukhendu Mandal

CrystEngComm, 2016, 18, 1046 

A series of metal–organic frameworks based on alkaline earth metal ions (Ca, Sr and Ba) and 1,3,5- benzenetribenzoic acid (BTB) have been synthesized and characterized. These are [H2NIJCH3)2]ijCa7IJBTB)5- IJH2O)8IJDMF)4]·4H2O (1), [H2NIJCH3)2]2ijSr5IJH2O)6IJBTB)4] (2) and [H2NIJCH3)2]ijBaIJH2O)IJBTB)] (3). All the structures are three-dimensional in nature with different secondary building units. Compound 1 contains one-dimensional Ca–O–Ca zigzag chains while compound 2 contains Sr5O28 pentameric clusters and compound 3 contains one-dimensional Ba–O–Ba chains. Both compounds 1 and 3 form (3,6)-net connectivity while compound 2 forms (3,12)-net connectivity. Optical band gap energy measurements show that compound 1 (2.65 eV) has low band gap energy compared to 2 (3.22 eV) and 3 (3.32 eV). This variation in band gap energy may be due to the difference in structural arrangement. Compound 3 crystallizes in a non-centrosymmetric space group (Pna21), which belongs to the polar point group C2v. This compound displays a strong SHG response and good ferroelectric and piezoelectric properties.

Self-oriented β-Crystalline Phase in Poly (vinylidene fluoride) Ferroelectric and Piezo-sensitive Ultra-thin Langmuir-Schaefer Film

Subrata Maji, Piyush Kanti Sarkar, Leena Aggarwal, Sujoy Kumar Ghosh, Dipankar Mandal, Goutam Sheet, Somobrata Acharya

Phys. Chem. Chem. Phys., 17, 8159-8165 (2015)

We report on the direct observation of ferroelectric switching and piezoelectric behaviour in ultrathin polyvinylidene fluoride (PVDF) films prepared by horizontal Langmuir-Schaefer (LS) technique. We have prepared pure β-phase by just increasing the number of LS layers without using additional non-ferroelectric assisting agent. Edge-on oriented CH2/CF2 units of PVDF at the air-water interface by means of hydrogen bonding network enable self-orientation of ferroelectric dipoles. Such restricted conformation of PVDF at the air-water interface results in the increase in net dipole moment with the number of LS layers. The ferroelectric switching and piezoelectric sensitivity are demonstrated by hysteretic polarization switching loops and butterfly-loops respectively. Successful circular domain writing on ultrathin LS film down to 5 monolayers of PVDF is demonstrated. Achievement of pure β-phase of PVDF at room temperature without using assisting agent may be found promising for non-volatile memory and piezoelectric based ultra thin smart sensor devices.

Ferroelectric-like response from the surface of SrTiO3 crystals at high temperatures

Shubhra Jyotsna, Ashima Arora, Jagmeet S. Sekhon, Goutam Sheet

Journal of Applied Physics 116, 104903 (2014)

 Since SrTiO3 has a high dielectric constant, it is used as a substrate for a large number of complex physical systems for electrical characterization. Since SrTiO3 crystals are known to be non-ferroelectric/non-piezoelectric at room temperature and above, SrTiO3 has been believed to be a good choice as a substrate/base material for PFM (Piezoresponse Force Microscopy) on novel systems at room temperature. In this paper, from PFM-like measurement using an atomic force microscope on bare crystals of (110) SrTiO3 we show that ferroelectric and piezoelectric-like response may originate from bare SrTiO3 at remarkably high temperatures up to 420 K. Electrical domain writing and erasing are also possible using a scanning probe tip on the surface of SrTiO3 crystals. This observation indicates that the role of the electrical response of SrTiO3 needs to be revisited in the systems where signature of ferroelectricity/piezoelectricity has been previously observed with SrTiO3 as a substrate/base material. (C) 2014 AIP Publishing LLC.

Synthesis and Characterization of Novel Azobenzene-based Mesogens and their organization at Air-water and Air-solid Interfaces

Monika Gupta, Nishtha Agarwal, Ashima Arora, Sandeep Kumar,Bharat Kumar, Goutam Sheet, Santanu Kumar Pal

RSC Advances 4, 41371-41377., (2014)

 Eight new oligomeric mesogens are reported consisting of an azobenzene-based core attached to which are four 4-cyanobiphenyl units via flexible alkyl spacers (n = 5-12). Their chemical structures were determined by 1H NMR, 13C NMR, IR, UV-Vis, Raman spectroscopy and elemental analysis. The thermotropic liquid crystalline properties of these materials were investigated by POM, DSC and XRD. The oligomers containing n = 8 and n = 10 were found to exhibit a monotropic nematic (N) phase while others were non-mesomorphic. Langmuir monolayer and Langmuir-Blodgett film of the nematic compound (n = 10) were studied at air-water interface (Langmuir film) and air-solid interface. Surface manometry studies on the Langmuir monolayer showed that the film had nearly zero surface pressure at large area per molecule (Am ≥ 0.55 nm2). The film on compression showed a gradual increase in surface pressure and finally collapsed at Am of about 0.15 nm2 with a collapse pressure of about 60 mN/m. Brewster angle microscopy (BAM) images (during compression) showed dark regions coexisting with grey spots at large area transformed to a uniform grey region with increase in surface density and finally collapsed that exhibited bright regions. Atomic force microscope studies (AFM) on LB films transferred onto freshly cleaved hydrophilic mica substrates exhibited network of thin fibers with height of fibers varying between ~4 nm to 80 nm that could be due to π-π stacking & dipolar interactions associated with the cyanobiphenyl units. On a hydrophobic silicon substrate, the LB transfer yielded a multilayer film which dewetted to form nanodroplets. We have carried out temperature dependent AFM of the nematic compound that showed the reversible formation of aligned fibers (~20-40 µm) in the mesophase. Overall, our study provides new approaches to the realization of controlling the anisotropic properties of the ordered phase.

Direct evidence of strong local ferroelectric ordering in a thermoelectric semiconductor

Leena Aggarwal, Jagmeet S. Sekhon, Satya N. Guin, Ashima Arora, Devendra S. Negi, Ranjan Datta, Kanishka Biswas, Goutam Sheet

Applied Physics Letters 105, 113903 (2014)

Add Description hereIt is thought that the proposed new family of multi-functional materials namely the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching we show that the recently discovered thermoelectric semiconductor $AgSbSe_{2}$ has local ferroelectric ordering. Using piezo-response force microscopy, we demonstrate the existence of nanometer scale ferroelectric domains that can be switched by external electric field. These observations are intriguing as $AgSbSe_{2}$ crystalizes in cubic rock salt structure with centro-symmetric space group (Fm-3m) and therefore no ferroelectricity is expected. However, from high resolution transmission electron microscopy (HRTEM) measurement we found the evidence of local superstructure formation which, we believe, leads to local distortion of the centro-symmetric arrangement in $AgSbSe_{2}$ and gives rise to the observed ferroelectricity. Stereochemically active $5s^{2}$ lone pair of Sb can also give rise to local structural distortion, which creates ferroelectricity in $AgSbSe_{2}$.

Voltage induced local hysteretic phase switching in silicon

Jagmeet Singh Sekhon, Leena Aggarwal, Goutam Sheet

Applied Physics Letters 104, 162908 (2014)

We report the observation of dc-bias induced 180 phase switching in silicon wafers by local-probe microscopy and spectroscopy. The switching is hysteretic and shows remarkable similarities with polarization switching in ferroelectrics as seen in piezoresponse force microscopy (PFM). This is always accompanied by a hysteretic amplitude vs. voltage curve which resembles the “butterfly loops” for piezoelectric materials. From a detailed analysis of the data obtained under different environmental and experimental conditions, we show that the hysteresis effects in phase and amplitude do not originate from ferro-electricity or piezoelectricity. This further indicates that mere observation of hysteresis effects in PFM does not confirm the existence of ferroelectric and/or piezoelectric ordering in materials. We also show that when samples are mounted on silicon for PFM measurements, the switching properties of silicon may appear on the sample even if the sample thickness is large.

Microscopic modulation of mechanical properties in transparent insect wings

Ashima Arora, Pramod Kumar, Jithin Bhagavathi, Kamal P. Singh, Goutam Sheet

Applied Physics Letters 104, 063702 (2014)

Abstract We report on the measurement of local friction and adhesion of transparent insect wings using an atomic force microscope cantilever down to nanometre length scales. We observe that the wing-surface is decorated with 10 μm long and 2 μm wide islands that have higher topographic height. The friction on the islands is two orders of magnitude higher than the back-ground while the adhesion on the islands is smaller. Furthermore, the high islands are decorated with ordered nano-wire-like structures while the background is full of randomly distributed granular nano-particles. Coherent optical diffraction through the wings produce a stable diffraction pattern revealing a quasi-periodic organization of the high islands over the entire wing. This suggests a long-range order in the modulation of friction and adhesion which is directly correlated with the topography. The measurements unravel novel functional design of complex wing surface and could find application in miniature biomimetic devices.

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Spectroscopic signature of two superconducting gaps and their unusual field dependence in RuB2

Soumya Datta, Aastha Vasdev, Soumyadip Halder, Jaskaran Singh, Yogesh Singh, and Goutam Sheet

arXiv:1907.08807v1 [cond-mat.supr-con] 20 Jul 2019

Recently RuB2 was shown to be a possible two-gap, type-I superconductor. Temperature dependent heat capacity measurements revealed a two-gap superconducting ground state while magnetic field dependent magnetization measurements indicated surprising type-I superconductivity with a very low experimental critical field (Hc) ∼ 120 Oe. In this paper, we report direct spectroscopic evidence of two superconducting energy gaps in RuB2. We have measured scanning tunnelling spectra exhibiting signature of two gaps on different grains of polycrystalline RuB2, possibly originating from multiple bands. Analysis of the temperature dependent tunnelling spectra revealed that the gaps from different bands evolve differently with temperature before disappearing simultaneously at a single Tc. Interestingly, our experiments also reveal that the gaps in quasiparticle density of states survive up to magnetic fields much higher than the bulk Hc and they evolve smoothly with field, unlike what is expected for a type-I superconductor, indicating the existence of a “mixed state”.

Spin-orbit driven spin depolarization in the ferromagnetic Weyl semimetal Co3Sn2S2

Sandeep Howlader, Surabhi Saha, Ritesh Kumar, Vipin Nagpal, Satyabrata Patnaik, Tanmoy Das and Goutam Sheet

arXiv:1906.06557v1 [cond-mat.supr-con] 15 Jun 2019

Co3Sn2S2 has recently emerged as a ferromagnetic Weyl semi-metal. Theoretical investigation of the spin-split bands predicted half metalicity in the compound. Here, we report the detection of a spin polarized supercurrent through a Nb/Co3Sn2S2 point contact where Andreev reflection is seen to be large indicating a large deviation from half metallicity. In fact, analysis of the Andreev reflection spectra reveals only 50% spin polarization at the Fermi level of Co3Sn2S2. Our theoretical calculations of electronic Density of States (DOS) reveal a spin depolarizing effect near the Fermi energy when the role of spin-orbit coupling is included. Inclusion of spin-orbit coupling also reveals particle-hole asymmetry that explains a large asymmetry observed in our experimental Andreev reflection spectra.

High spin-polarization in the low Curie temperature complex itinerant ferromagnet EuTi1−xNbxO3

Suman Kamboj, Deepak K. Roy, Susmita Roy, Rajeswari Roy Chowdhury, Prabhat Mandal, Mukul Kabir and Goutam Sheet

arXiv:1903.12022v1 [cond-mat.mtrl-sci] 28 Mar 2019

The physical systems with ferromagnetism and “bad” metallicity hosting unusual transport properties are playgrounds of novel quantum phenomena. Recently EuTi1−xNbxO3 emerged as a ferromagnetic system where non-trivial temperature dependent transport properties are observed due to coexistence and competition of various magnetic and non-magnetic scattering processes. In the ferromagnetic state, the resistivity shows a T 2 temperature dependence possibly due to electronmagnon scattering and above the Curie temperature Tc, the dependence changes to T 3/2 behaviour indicating a correlation between transport and magnetic properties. In this paper, we show that the transport spin-polarization in EuTi1−xNbxO3, a low Curie temperature ferromagnet, is as high (∼ 40%) as that in some of the metallic ferromagnets with high Curie temperatures. In addition, owing to the low Curie temperature of EuTi1−xNbxO3, the temperature (T) dependence of Pt could be measured systematically up to Tc which revealed a proportionate relationship with magnetization Ms vs. T. This indicates that such proportionality is far more universally valid than the ferromagnets with ideal parabolic bands. Furthermore, our band structure calculations not only helped understand the origin of such high spin polarization in EuTi1−xNbxO3 but also provided a route to estimate the Hubbard U parameter in complex metallic ferromagnets in general using experimental inputs

Enhanced, homogeneous type II superconductivity in Cu-intercalated PdTe2

Aastha Vasdev, Anshu Sirohi, M. K. Hooda, C. S. Yadav, and Goutam Sheet

arXiv:1903.04321v1 [cond-mat.supr-con] 11 Mar 2019

Though the superconducting phase of the type-II Dirac semimetal PdTe2 was shown to be conventional in nature, the phase continued to be interesting in terms of its magnetic properties. While certain experiments indicated an unexpected type-I superconducting phase, other experiments revealed formation of vortices under the application of magnetic fields. Recently, scanning tunneling spectroscopy (STS) experiments revealed the existence of a mixed phase where type-I and type-II behaviours coexist. Here, based on our temperature and magnetic field dependent STS experiments on Cu-intercalated PdTe2 we show that as the critical temperature of the superconducting phase goes up from 1.7 K to 2.4 K on Cu-intercalation, the mixed phase disappears and the system becomes homogeneously type-II. This may be attributed to an averaging effect caused by quasiparticle exchange between type-I and type-II domains mediated by the Cu atoms and to decreased coherence length due to increased disorder.

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