arXiv subjects

Anand Kamlapure

Publications and source records attributed to Anand Kamlapure.

At least 19 recordsLinked to original sources

arXiv2022

Threefold enhancement of superconductivity and the role of field-induced odd-frequency pairing in epitaxial aluminum films near the 2D limit

BCS theory has been widely successful at describing elemental bulk superconductors. Yet, as the length scales of such superconductors approach the atomic limit, dimensionality as well as the environment of the superconductor can lead to drastically different and unpredictable superconducting behavior. Here, we report a threefold enhancement of the superconducting critical temperature and gap size in ultrathin epitaxial Al films on Si(111), when approaching the 2D limit, based on high-resolution scanning tunneling microscopy/spectroscopy (STM/STS) measurements. In magnetic field, the Al films show type II behavior and the Meservey-Tedrow-Fulde (MTF) effect for in-plane magnetic fields. Using spatially resolved spectroscopy, we characterize the vortex structure in the MTF regime and find strong deviations from the typical Abrikosov vortex. We corroborate these findings with calculations that unveil the role of odd-frequency pairing and a paramagnetic Meissner effect. These results illustrate two striking influences of reduced dimensionality on a BCS superconductor and present a new platform to study BCS superconductivity in large magnetic fields.

cond-mat.supr-con↗

arXiv2021

Anisotropic superconductivity induced at a hybrid superconducting-semiconducting interface

Epitaxial semiconductor-superconductor heterostructures are promising as a platform for gate-tunable superconducting electronics. Thus far, the superconducting properties in such hybrid systems have been predicted based on simplified hybridization models which neglect the electronic structure that can arise at the interface. Here, we demonstrate that the hybrid electronic structure derived at the interface between semiconducting black phosphorus and atomically thin films of lead can drastically modify the superconducting properties of the thin metallic film. Using ultra-low temperature scanning tunneling microscopy and spectroscopy, we ascertain the moiré structure driven by the interface, and observe a strongly anisotropic renormalization of the superconducting gap and vortex structure of the lead film. Based on density functional theory, we attribute the renormalization of the superconductivity to weak hybridization at the interface where the anisotropic characteristics of the semiconductor band structure is imprinted on the Fermi surface of the superconductor. Based on a hybrid two-band model, we link this hybridization-driven renormalization to a weighting of the superconducting order parameter that quantitatively reproduces the measured spectra. These results illustrate the effect of interfacial hybridization at superconductor-semiconductor heterostructures, and pathways for engineering quantum technologies based on gate-tunable superconducting electronics.

cond-mat.supr-con↗

arXiv2021

Moiré induced electronic structure modifications in monolayer V$_{2}$S$_{3}$ on Au(111)

There is immense interest in how the local environment influences the electronic structure of materials at the single layer limit. We characterize moiré induced spatial variations in the electronic structure of in-situ grown monolayer V2S3 on Au(111) by means of low temperature scanning tunneling microscopy and spectroscopy. We observe a long-range modulation of the integrated local density of states (LDOS), and quantify this modulation with respect to the moiré superstructure for multiple orientations of the monolayer with respect to the substrate. Scanning tunneling spectroscopy reveals a prominent peak in the LDOS, which is shifted in energy at different points of the moiré superstructure. Comparing ab initio calculations with angle-resolved photoemission, we are able to attribute this peak to bands that exhibit a large out-of-plane d-orbital character. This suggests that the moiré driven variations in the measured density of states is driven by a periodic modulation of the monolayer-substrate hybridization.

cond-mat.mtrl-sci↗

arXiv2018

Engineering the spin couplings in atomically crafted spin chains on an elemental superconductor

Magnetic atoms on a superconductor give rise to Yu-Shiba-Rusinov (YSR) states within the superconducting energy gap. A spin chain of magnetic adatoms on an s-wave superconductor may lead to topological superconductivity accompanied by the emergence of Majorana modes at the chain ends. For their usage in quantum computation, it is a prerequisite to artificially assemble the chains and control the exchange couplings between the spins in the chain and in the substrate. Here, using a scanning tunneling microscope tip, we demonstrate engineering of the energy levels of the YSR states by placing interstitial Fe atoms in close proximity to adsorbed Fe atoms on an oxidized Ta surface. Based on this prototype platform, we show that the interaction within a long chain can be strengthened by linking the adsorbed Fe atoms with the interstitial ones. Our work adds an important step towards the controlled design and manipulation of Majorana end states.

cond-mat.supr-con↗

arXiv2017

Enhanced spin ordering temperature in ultrathin FeTe films grown on a topological insulator

We studied the temperature dependence of the diagonal double-stripe spin order in one and two unit cell thick layers of FeTe grown on the topological insulator Bi_2Te_3 via spin-polarized scanning tunneling microscopy. The spin order persists up to temperatures which are higher than the transition temperature reported for bulk Fe_1+yTe with lowest possible excess Fe content y. The enhanced spin order stability is assigned to a strongly decreased y with respect to the lowest values achievable in bulk crystal growth, and effects due to the interface between the FeTe and the topological insulator. The result is relevant for understanding the recent observation of a coexistence of superconducting correlations and spin order in this system.

cond-mat.str-el↗

arXiv2016

Evidence for a two-fold symmetric superconducting gap in a monolayer of FeSe$_{0.5}$Te$_{0.5}$ on a topological insulator

We present our investigations on the superconducting properties of monolayers of FeSe$_{0.5}$Te$_{0.5}$ grown on the 3D topological insulator Bi$_{2}$Se$_{1.2}$Te$_{1.8}$ using low temperature scanning tunneling spectroscopy (STS). While the morphology and the overall transition temperature resemble those of similarly doped bulk crystals, the spatially resolved spectroscopic data at 1.1K shows a much larger spatial inhomogeneity in the superconducting energy gaps. Despite the gap inhomogeneity all the spectra can be fitted with a two-fold anisotropic s-wave gap function. The two-fold nature of the gap symmetry is evident from the Bogoliubov quasiparticle interference (QPI) pattern which shows distinct C$_{2}$ symmetric scattering intensities. We argue that the gap inhomogeneity emerges as a result of intrinsic disorder in our system similar to disordered conventional superconductors. Even though most of our findings clearly differ from the current understanding of the corresponding bulk system, it provides an ideal platform to study unconventional superconductivity in Fe chalcogenides thinned down to a single layer and in close proximity to a topological insulator.

cond-mat.supr-con↗

arXiv2016

Reorientation of the bicollinear antiferromagnetic structure at the surface of Fe$_{1+y}$Te bulk and thin films

Establishing the relation between the ubiquitous antiferromagnetism in the non-superconducting parent compounds of unconventional superconductors and their superconducting phase is believed to be important for the understanding of the complex physics in these materials. Going from the bulk systems to thin films strongly affects the phase diagram of unconventional superconductors. For Fe$_{1+y}$Te, the parent compound of the Fe$_{1+y}$Se$_{1-x}$Te$_x$ superconductors, bulk sensitive neutron diffraction has revealed an in-plane oriented bicollinear antiferromagnetic structure. Here, we show by spin-resolved scanning tunneling microscopy that on the surfaces of bulk Fe$_{1+y}$Te, as well as on thin films grown on the topological insulator Bi$_2$Te$_3$, the spin direction is canted both away from the surface plane and from the high-symmetry directions of the surface unit cell, while keeping the bicollinear magnetic structure. Our results demonstrate that the magnetism at the Fe-chalcogenide surface markedly deviates from a simple in-plane oriented bicollinear antiferromagnetic structure, which implies that the pairing at the surface of the related superconducting compounds might be different from that in the bulk.

cond-mat.str-el↗

arXiv2016

Evidence for interfacial superconductivity in a bi-collinear antiferromagnetically ordered FeTe monolayer on a topological insulator

The discovery of high-temperature superconductivity in Fe-based compounds [1,2] has triggered numerous investigations on the interplay between superconductivity and magnetism [3] and, more recently, on the enhancement of transition temperatures through interface effects [4]. It is widely believed that the emergence of optimal superconductivity is intimately linked to the suppression of long-range antiferromagnetic (AFM) order, although the exact microscopic picture of this relationship remains elusive [1] due to the lack of data with atomic spatial resolution [5-7]. Here, we present a spin-polarized scanning tunneling spectroscopy (SP-STS) study of ultrathin FeTe$_{1-x}$Se$_x$ (x = 0, 0.5) films grown on prototypical Bi-based bulk topological insulators. Surprisingly, we find an energy gap at the Fermi level indicating superconducting correlations up to Tc ~ 6 K for one unit cell thin FeTe layers grown on Bi2Te3 substrates, in contrast to the non-superconducting FeTe bulk compound [8]. Moreover, SP-STS reveals that the energy gap spatially coexists with bicollinear AFM order. This finding opens novel perspectives for theoretical studies of competing orders in Fe-based superconductors as well as for experimental investigations of exotic phases in heterostructures of topological insulators and superconducting layers.

cond-mat.supr-con↗

arXiv2015

Real space imaging of metastable Bragg glass states in a weakly pinned Type II superconductor

The breakdown of crystalline order in a disordered background connects to some of the most challenging problems in condensed matter physics. For a superconducting vortex lattice, the equilibrium state in the presence of impurities is predicted to be a "Bragg glass" (BG), where the local crystalline order is maintained everywhere and yet the global positional order decays algebraically. Here, using scanning tunnelling spectroscopy (STS) we image the vortex lattice in a weakly pinned NbSe2 single crystal. We present direct evidence that the ordered state of the VL is a BG, consisting of a large number of degenerate metastable states, which is a hallmark of a glassy state. These results are a significant step towards understanding the disordering of a lattice under the influence of quenched random disorder with a direct impact on various fields, including charge density waves, colloidal crystals and self-organised periodic structures on a substrate.

cond-mat.supr-con↗

arXiv2013

A 350 mK, 9 T scanning tunneling microscope for the study of superconducting thin films and single crystals

We report the construction and performance of a low temperature, high field scanning tunneling microscope (STM) operating down to 350mK and in magnetic fields up to 9T, with thin film deposition and in-situ cleaving capabilities. The main focus lies on the simple design of STM head allowing top loading of the sample and innovative sample holder design that allows us to get spectroscopic data on superconducting thin films grown in-situ on insulating substrates. Other design details on sample transport, chamber and vibration isolation scheme are also described. We demonstrate the capability of our instrument through the atomic resolution imaging and spectroscopy on NbSe2 single crystal and spectroscopic maps obtained on homogeneously disordered NbN thin film.

cond-mat.supr-con↗

arXiv2013

Emergence of nanoscale inhomogeneity in the superconducting state of a homogeneously disordered conventional superconductor, NbN

The notion of spontaneous formation of an inhomogeneous superconducting state is at the heart of most theories attempting to understand the superconducting state in the presence of strong disorder. Using scanning tunneling spectroscopy and high resolution scanning transmission electron microscopy, we experimentally demonstrate that under the competing effects of strong homogeneous disorder and superconducting correlations, the superconducting state of a conventional superconductor, NbN, spontaneously segregates into domains. Tracking these domains as a function of temperature we observe that the superconducting domains persist across the bulk superconducting transition, Tc, and disappear close to the pseudogap temperature, T*, where signatures of superconducting correlations disappear from the tunneling spectrum and the superfluid response of the system.

cond-mat.supr-con↗

arXiv2012

Frequency dependent superfluid stiffness in the pseudogap regime in strongly disordered NbN thin films

We measure the frequency dependence of the complex ac conductivity of NbN films with different levels of disorder in frequency range 0.4-20 GHz. Films with low disorder exhibit a narrow dynamic fluctuation regime above T_c as expected for a conventional superconductor. However, for strongly disordered samples, the fluctuation regime extends well above T_c, with a strongly frequency-dependent superfluid stiffness which disappears only at a temperature T* close to the pseudogap temperature obtained from scanning tunneling measurements. Such a finite-frequency response is associated to a marked slowing down of the superconducting fluctuations already below T*. The corresponding large length-scale fluctuations suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.

cond-mat.supr-con↗

arXiv2012

Observation of Andreev bound state and multiple energy gaps in the non-centrosymmetric superconductor BiPd

We report directional point contact Andreev reflection (PCAR) measurements on high-quality single crystals of the non-centrosymmetric superconductor, BiPd. The PCAR spectra measured on different crystallographic faces of the single crystal clearly show the presence of multiple superconducting energy gaps. For point contacts with low resistance, in addition to the superconducting gap feature, a pronounced zero bias conductance peak is observed. These observations provide strong evidence of the presence of unconventional order parameter in this material.

cond-mat.supr-con↗

arXiv2011

Phase diagram of a strongly disordered s-wave superconductor, NbN, close to the metal-insulator transition

We present a phase diagram as a function of disorder in three-dimensional NbN thin films, as the system enters the critical disorder for the destruction of the superconducting state. The superconducting state is investigated using a combination of magnetotransport and tunneling spectroscopy measurements. Our studies reveal 3 different disorder regimes. At low disorder the (k_{F}l~10-4), the system follows the mean field Bardeen-Cooper-Schrieffer behavior where the superconducting energy gap vanishes at the temperature where electrical resistance appears. For stronger disorder (k_{F}l<4) a "pseudogap" state emerges where a gap in the electronic spectrum persists up to temperatures much higher than Tc, suggesting that Cooper pairs continue to exist in the system even after the zero resistance state is destroyed. Finally, very strongly disordered samples (k_{F}l<1) exhibit a pronounced magnetoresistance peak at low temperatures, suggesting that localized Cooper pairs continue to survive in the system even after the global superconducting ground state is completely destroyed.

cond-mat.supr-con↗

arXiv2011

Role of the vortex-core energy on the Beresinkii-Kosterlitz-Thouless transition in thin films of NbN

We analyze the occurrence of the Beresinkii-Kosterlitz-Thouless transition in thin films of NbN at various film thickness, by probing the effect of vortex fluctuations on the temperature dependence of the superfluid density below $T_{BKT}$ and of the resistivity above $T_{BKT}$. By direct comparison between the experimental data and the theory we show the crucial role played by the vortex-core energy in determining the characteristic signatures of the BKT physics, and we estimate its dependence on the disorder level. Our work provides a paradigmatic example of BKT physics in a quasi-two-dimensional superconductor.

cond-mat.supr-con↗

arXiv2010

Phase fluctuations in a strongly disordered s-wave superconductor close to the metal-insulator transition

We explore the role of phase fluctuations in a 3-dimensional s-wave superconductor, NbN, as we approach the critical disorder for the destruction of the superconducting state. Scanning tunneling spectroscopy measurements reveal that in the presence of strong disorder, a finite gap in the electronic spectrum continues to persist at temperatures above Tc. With increase in disorder, the superfluid density is strongly suppressed at low temperatures and evolves towards a linear-T variation at higher temperatures. These observations provide strong evidence that phase fluctuation leads to the formation of a pseudogap state in a disordered conventional superconductor.

cond-mat.supr-con↗

arXiv2010

Phase diagram and upper critical field of homogenously disordered epitaxial 3-dimensional NbN films

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal-insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder we observe that as kFl-->1 the superconducting transition temperature (Tc) and minimum conductivity (sigma_0) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal-insulator transition and the superconductor-insulator transition occur at a single quantum critical point at kFl~1.

cond-mat.supr-con↗

arXiv2010

Effect of Phase Fluctuations on the Superconducting Properties of Strongly Disordered 3D NbN Thin Films

We present transport, Hall effect, electronic tunnelling and penetration depth studies in 3D homogeneously disordered epitaxial NbN thin films with disorder ranging from the moderately clean limit (k_Fl~10.12) to the very dirty limit (k_Fl~1.24). The superconducting transition temperature (T_c) decreases from ~17K to less than 350mK with increasing disorder. The T_c and conductivity at the lowest temperature both asymptotically approach zero as k_Fl tends to 1, indicating a coincidence of the metal-insulator transition (MIT) and the superconductor-insulator transition. Close to critical disorder there is spatial inhomogeneity in the superconducting density of states (DOS) and the superconducting state is governed by quantum phase fluctuations. This results in suppression of the superfluid density (n_s) and a pseudogap state where the resistance is no longer zero but the energy gap remains finite

cond-mat.supr-con↗