arXiv subjects

Goutam Sheet

Publications and source records attributed to Goutam Sheet.

At least 37 records · Page 2Linked to original sources

arXiv2022

Observation of Weak Anti-localization in thin films of the Topological Semimetal Candidate PdSb$_{2}$

We report results of a magneto-transport study on thin films of the topological semi-metal candidate PdSb$_{2}$ (PS). We observe a positive correction to magneto-conductivity at low temperatures, which is a signature of weak anti-localization (WAL). We analyze the WAL data within the Hikami-Larkin-Nagaoka (HLN) theory and extract the dephasing length (L$_ϕ$) whose temperature dependence reveals the various phase relaxation mechanisms. From the WAL effect, we also extract $α$ (the number of transport channels). The evolution of $α$ with temperature and film thickness reflects how the coupling between different conducting channels (Topological surface channels and bulk channels) changes. Additionally, the electron-electron interaction (EEI) effect was observed in the temperature-dependent conductivity at low temperatures. From the EEI effect, we get an alternate estimate of the number of transport channels and obtain a value similar to that obtained from the analysis of the WAL effect. This suggests that in disordered films, the EEI effect can be used to get information about the coupling of the topological surface states with each other and with bulk states.

cond-mat.str-el↗

arXiv2021

Superconductivity coexisting with ferromagnetism in a quasi-one dimensional non-centrosymmetric (TaSe$_4$)$_3$I

Low-dimensional materials with broken inversion symmetry and strong spin-orbit coupling can give rise to fascinating quantum phases and phase transitions. Here we report coexistence of superconductivity and ferromagnetism below 2.5\,K in the quasi-one dimensional crystals of non-centrosymmetric (TaSe$_4$)$_3$I (space group: $P\bar{4}2_1c$). The unique phase is a direct consequence of inversion symmetry breaking as the same material also stabilizes in a centro-symmetric structure (space group: $P4/mnc$) where it behaves like a non-magnetic insulator. The coexistence here upfront contradicts the popular belief that superconductivity and ferromagnetism are two apparently antagonistic phenomena. Notably, here, for the first time, we have clearly detected Meissner effect in the superconducting state despite the coexisting ferromagnetic order. The coexistence of superconductivity and ferromagnetism projects non-centrosymmetric (TaSe$_4$)$_3$I as a host for complex ground states of quantum matter including possible unconventional superconductivity with elusive spin-triplet pairing.

cond-mat.supr-con↗

arXiv2021

Andreev reflection spectroscopy on SnAs single crystals

Binary compounds of SnT family(T= As, Sb, Te, S, Se, P) exhibit novel properties like superconductivity, topologically protected states etc. Some of these compounds crystallize in NaCl type structure or in layered structure, both of which are considered potentially important for high-temperature/unconventional superconductivity. It was previously shown that K-doped BaBiO$_3$ with NaCl type structure exhibits superconducting state below $\sim$ 30 K. SnAs has crystallographic configuration that is exactly similar to K-doped BaBiO$_3$ and this warrants for the investigation of superconductivity in this material. Previously it was reported that SnAs exhibits weakly coupled type I superconductivity with an energy gap of 0.7 meV. Recently, the electronic bandstructure calculations has hinted to the existence of possible topologically protected states in SnAs. This has motivated us to investigate the superconducting nature of SnAs using point contact Andreev reflection spectroscopy. Our investigation revealed that superconductivity in SnAs can be well explained within the BCS framework in the weak coupling limit.

cond-mat.supr-con↗

arXiv2021

Superconductivity in silicon: a review

Silicon, one of the most abundant elements found on Earth, has been an excellent choice of the semiconductor industry for ages. Despite it's remarkable applications in modern semiconductor-based electronic devices, the potential of cubic silicon in superconducting electronics remained a challenge because even heavily doped silicon crystals do not superconduct under normal conditions. It is apparent that if superconductivity can be introduced in cubic silicon, that will bring a breakthrough in low-dissipation electronic circuitry. Motivated by this, attempts have been made by several research groups to induce superconductivity in silicon through a number of different routes. Some of the other structural phases of silicon like $β$-Sn and simple hexagonal are, however, known to display superconductivity. In the present review article, various theoretical and experimental aspects of superconductivity in silicon are discussed. Superconductivity in different phases and different structural forms of silicon are also reviewed. We also highlight the potential of superconducting phases of silicon for technological applications in super-conducting nano-electronics.

cond-mat.supr-con↗

arXiv2021

Enhanced electrical transport through wrinkles in turbostratic graphene films

Formation of wrinkles is a common phenomenon in the large area growth of two dimensional layered materials on metallic substrates. Wrinkles can significantly affect the working of 2D materials based large scale electronic devices and therefore, it is of utmost importance to investigate local electrical properties of such wrinkled/folded structures on 2D materials. Here, we report local conductivity measurements by conducting atomic force microscopy (CAFM) and surface potential mapping by Kelvin Probe Force microscopy (KPFM) on large area wrinkled turbostratic graphene films grown on nickel foils. We show that the electrical transport current is several orders of magnitude higher on the wrinkles than that on the flat regions of the graphene films. Therefore, our results suggest that controlled engineering of such wrinkles on graphene may facilitate development of superior graphene-based nano-electronic devices where transport of high current through narrow channels are desired.

cond-mat.mes-hall↗

arXiv2021

Field induced hysteretic structural phase switching and possible CDW in Re-doped MoTe$_2$

Novel electronic systems displaying exotic physical properties can be derived from complex topological materials through chemical doping. MoTe$_2$, the candidate type-II Weyl semimetal shows dramatically enhanced superconductivity up to 4.1 K upon Re doping in Mo sites. Based on bulk transport and local scanning tunneling microscopy (STM) here we show that Re doping also leads to the emergence of a possible charge density wave (CDW) phase in Re$_{0.2}$Mo$_{0.8}$Te$_2$. In addition, the tunneling $I-V$ characteristics display non-linearity and hysteresis which is commensurate with a hysteresis observed in the change in tip-height ($z$) as a function of applied voltage $V$. The observations indicate an electric field induced hysteretic switching consistent with piezoelectricity and possible ferroelectricity.

cond-mat.str-el↗

arXiv2021

Spectroscopic evidence of mixed angular momentum symmetry in non-centrosymmetric Ru$_7$B$_3$

Superconducting crystals with lack of inversion symmetry can potentially host unconventional pairing. However, till date, no direct conclusive experimental evidence of such unconventional order parameters in non-centrosymmetric superconductors has been reported. In this paper, through direct measurement of the superconducting energy gap by scanning tunnelling spectroscopy, we report the existence of both $s$-wave (singlet) and $p$-wave (triplet) pairing symmetries in non-centrosymmetric Ru$_7$B$_3$. Our temperature and magnetic field dependent studies also indicate that the relative amplitudes of the singlet and triplet components of the order parameter change differently with temperature.

cond-mat.supr-con↗

arXiv2021

Odd-even rule for zero-bias tunneling conductance in coupled Majorana wire arrays

A semiconducting nanowire with strong Rashba coupling and in proximity of a superconductor hosts Majorana edge modes. An array of such nanowires with inter-wire coupling gives an approximate description of a two-dimensional topological superconductor, where depending on the strength of the magnetic field and the chemical potential, a rich phase diagram hosting trivial and different types of non-trivial phases can be achieved. Here, we theoretically consider such a two-dimensional assembly of spin-orbit coupled superconducting nanowires and calculate the collective tunneling conductance between normal electrodes and the wires in the topological regime. When the number of wires in the assembly is $N$, as a consequence of the way the Majorana bonding and anti-bonding states form, we find that $N$ conductance peaks symmetric about the bias $V = 0$ appear, for even $N$. When $N$ is odd, a ZBCP also appears. Such an assembly can be realized by standard nano-fabrication techniques where individual nanowires can be turned $ON$ or $OFF$ by using mechanical switch (or local top gating) to make $N$ either even or odd -- thereby switching the ZBCP $OFF$ or $ON$, respectively. Hence, our results can be used to realize and detect topological superconductivity efficiently, unambiguously and in a controlled manner.

cond-mat.mes-hall↗

arXiv2020

Weak Anti-localization in thin films of the Topological Semimetal Candidate Pd$_{3}$Bi$_{2}$S$_{2}$

We report the growth and magneto-transport studies of Pd$_{3}$Bi$_{2}$S$_{2}$ (PBS) thin films synthesized by pulsed laser deposition (PLD) technique. The magneto-transport study on pristine and post annealed films show the presence of more than one type of charge carrier with a carrier concentration in the range $0.6$ - $2.26~\times$ 10$^{21}$ cm$^{-3}$ and mobility in the range 0.96 - 1.73 $\times$ 10$^{2}$ cm$^{2}$/Vs. At low temperatures a logarithmic increase in conductivity is observed which indicates the presence of weak anti-localization (WAL). The magnetotransport data is analysed within the Hikami-Larkin-Nagaoka (HLN) theory. It is found that temperature dependence of the dephasing length can't be explained only by electron-electron scattering and that electron-phonon scattering also contributes to the phase relaxation mechanism in PBS films.

cond-mat.str-el↗

arXiv2020

Tip-induced Superconductivity

It is widely believed that topological superconductivity, a hitherto elusive phase of quantum matter, can be achieved by inducing superconductivity in topological materials. In search of such topological superconductors, certain topological insulators (like, Bi$_2$Se$_3$) were successfully turned into superconductors by metal-ion (Cu, Pd, Sr, Nb etc. ) intercalation. Superconductivity could be induced in topological materials through applying pressure as well. for example, a pressure-induced superconducting phase was found in the topological insulator Bi$_2$Se$_3$. However, in all such cases, no conclusive signature of topological superconductivity was found. In this review, we will discuss about another novel way of inducing superconductivity in a non-superconducting topological material -- by creating a mesoscopic interface on the material with a non-superconducting, normal metallic tip where the mesoscopic interface becomes superconducting. Such a phase is now known as a tip-induced superconducting (TISC) phase. This was first seen in 2014 on Cd$_3$As$_2$ at IISER Mohali, India. Following that, a large number of other topological materials were shown to display TISC. Since the TISC phase emerges only at a confined region under a mesoscopic point contact, traditional bulk tools for characterizing superconductivity cannot be employed to detect/confirm such a phase. On the other hand, such a point contact geometry is ideal for probing the possible existence of a temperature and magnetic field dependent superconducting energy gap and a temperature and magnetic field dependent critical current. We will review the details of the experimental signatures that can be used to prove the existence of superconductivity even when the "text-book" tests for detecting superconductivity cannot be performed. Then, we will review different systems where a TISC phase could be realized.

cond-mat.supr-con↗

arXiv2020

Non-ballistic transport characteristics of superconducting point-contacts

In the "ballistic" regime, the transport across a normal metal (N)/superconductor (S) point-contact is dominated by a quantum process called Andreev reflection. Andreev reflection causes an enhancement of the conductance below the superconducting energy gap, and the ratio of the low-bias and the high-bias conductance cannot be greater than 2 when the superconductor is conventional in nature. In this regime, the features associated with Andreev reflection also provide energy and momentum-resolved spectroscopic information about the superconducting phase. Here we theoretically consider various types of N/S point contacts, away from the ballistic regime, and show that even when the superconductor under investigation is simple conventional in nature, depending on the shape, size and anatomy of the point contacts, a wide variety of spectral features may appear in the conductance spectra. Such features may misleadingly mimic theoretically expected signatures of exotic physical phenomena like Klein tunneling in topological superconductors, Andreev bound states in unconventional superconductors, multiband superconductivity and Majorana zero modes.

cond-mat.supr-con↗

arXiv2020

Optoelectronic characterisation of twisted germanium sulfide nanowires with experimental observation of intrinsic ferroelectricity

We report the optoelectrical characterisation of Eshelby twisted Germanium sulfide (GeS) nanowires with first experimental observation of ferroelectric order at room temperature in GeS which is an otherwise centrosymmetric molecule. The chemical composition and structure of these nanowires were confirmed by various spectroscopic, microscopic and diffractive techniques. In addition, the nanowires were found to be stable over time. From the optoelectronic measurements we found that these p-type semiconducting GeS nanowires have up to two orders higher charge carrier mobility than GeS nanosheets and have sustainable, robust photo-switching property. The existence of room temperature ferroelectricity is confirmed by piezoresponse force microscopy which showed hysteresis and butterfly loop, characteristics of a ferroelectric material. Our observations reveal that the properties of twisted GeS nanowires can be harnessed in making efficient electric sensors, photodetectors, data memories and flexible electronics.

cond-mat.mes-hall↗

arXiv2020

The pressure-enhanced superconducting phase of Sr$_x$-Bi$_2$Se$_3$ probed by hard point contact spectroscopy

The superconducting systems emerging from topological insulators upon metal ion intercalation or application of high pressure are ideal for investigation of possible topological superconductivity. In this context, Sr-intercalated Bi$_2$Se$_3$ is specially interesting because it displays pressure induced re-entrant superconductivity where the high pressure phase shows almost two times higher $T_c$ than the ambient superconducting phase ( $T_C\sim$ 2.9 K). Interestingly, unlike the ambient phase, the pressure-induced superconducting phase shows strong indication of unconventional superconductivity. However, since the pressure-induced phase remains inaccessible to spectroscopic techniques, the detailed study of the phase remained an unattained goal. Here we show that the high-pressure phase can be realized under a mesoscopic point contact, where transport spectroscopy can be used to probe the spectroscopic properties of the pressure-induced phase. We find that the point contact junctions on the high-pressure phase show unusual response to magnetic field supporting the possibility of unconventional superconductivity.

cond-mat.supr-con↗

arXiv2020

Domain structure dynamics in the ferromagnetic Kagome-lattice Weyl semimetal Co$_3$Sn$_2$S$_2$

Co$_3$Sn$_2$S$_2$, a Weyl semimetal that consists of layers of Kagome lattices, transitions from a high-temperature paramagnetic phase to a low-temperature ferromagnetic phase below 177 K. The phase transition occurs through an intermediate non-trivial magnetic phase, the so-called "A"-phase just below the Curie temperature. The "A"-phase was earlier linked with a competing anti-ferromagnetic phase, a spin-glass phase and certain indirect measurements indicated the possibility of magnetic Skyrmions in this phase. We have imaged the magnetic domain structure in a single crystal of Co$_3$Sn$_2$S$_2$ at different temperatures, magnetic fields and field-angles by magnetic force microscopy. At low temperatures, we observed stripe domains indicating presence of uniaxial anisotropy. Above 130 K, the domain walls become mobile and they tend to align relatively easily when the magnetic field is increased along the $c$-axis than in the $a-b$ plane. Our detailed study of field-dependent domain dynamics reveal that the anomalous nature of the phase transition just below $T_c$ is dominantly governed by domain wall motion.

cond-mat.mtrl-sci↗

arXiv2019

Spectroscopic signature of two superconducting gaps and their unusual field dependence in RuB$_2$

Recently RuB$_2$ 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 ($H_c$) $\sim$ 120 Oe. In this paper, we report direct spectroscopic evidence of two superconducting energy gaps in RuB$_2$. We have measured scanning tunnelling spectra exhibiting signature of two gaps on different grains of polycrystalline RuB$_2$, 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 $T_c$. Interestingly, our experiments also reveal that the gaps in quasiparticle density of states survive up to magnetic fields much higher than the bulk $H_c$ and they evolve smoothly with field, unlike what is expected for a type-I superconductor, indicating the existence of a ``mixed state".

cond-mat.supr-con↗

arXiv2019

Comment on "Perfect Andreev reflection due to the Klein paradox in a topological superconducting state (Nature \textbf{570}, 344 (2019))"

In a recent publication, Lee $et$ $al.$ discussed experimental observation of Klein tunneling into a proximity-induced topological superconducting state of SmB$_{6}$ through the measurement of Andreev reflection with low-bias conductance doubling across point-contact junctions between sharp Pt-Ir tips and SmB$_{6}$/YB$_{6}$ heterostructures. However, the interpretation of the presented point-contact data is rather ambiguous because observing a low-bias conductance enhancement by a factor of 2 is not special for Klein tunneling into topological superconductors. Such an enhanced conductance can be observed in point contacts between all types of superconductors and normal metals when the contacts are neither in the ballistic nor in the diffusive regimes of mesoscopic transport, but in a dissipative thermal regime. The thermal regime is expected for the point contacts presented in Lee $et$ $al.$ primarily because of the anticipated very short electron mean free path of SmB$_6$ -- we will discuss this in more detail here.

cond-mat.supr-con↗

arXiv2019

A modular point contact spectroscopy probe for sub-Kelvin applications

We present the construction and performance of a plug-n-play type point contact spectroscopy probe equipped with a piezo-driven 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 Tesla 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 a few elemental superconductors with low T$_{c}$. We have also used the probe to perform spectroscopic measurements on another low temperature superconductor PdTe$_2$, 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 tunnelling spectroscopy etc.

cond-mat.supr-con↗

arXiv2019

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

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.

cond-mat.mes-hall↗