arXiv subjects

Leena Aggarwal

Publications and source records attributed to Leena Aggarwal.

12 recordsLinked to original sources

arXiv2023

Atomic-Scale Visualization of a Cascade of Magnetic Orders in the Layered Antiferromagnet $GdTe_{3}$

$GdTe_{3}$ is a layered antiferromagnet which has attracted attention due to its exceptionally high mobility, distinctive unidirectional incommensurate charge density wave (CDW), superconductivity under pressure, and a cascade of magnetic transitions between 7 and 12 K, with as yet unknown order parameters. Here, we use spin-polarized scanning tunneling microscopy to directly image the charge and magnetic orders in $GdTe_{3}$. Below 7 K, we find a striped antiferromagnetic phase with twice the periodicity of the Gd lattice and perpendicular to the CDW. As we heat the sample, we discover a spin density wave with the same periodicity as the CDW between 7 and 12 K; the viability of this phase is supported by our Landau free energy model. Our work reveals the order parameters of the magnetic phases in $GdTe_{3}$ and shows how the interplay between charge and spin can generate a cascade of magnetic orders.

cond-mat.str-el↗

arXiv2023

Influence of atomic relaxations on the moir\'{e} flat band wavefunctions in antiparallel twisted bilayer WS$_{\text{2}}$

Twisting bilayers of transition metal dichalcogenides (TMDs) gives rise to a periodic moir\'{e} potential resulting in flat electronic bands with localized wavefunctions and enhanced correlation effects. In this work, scanning tunneling microscopy is used to image a WS$_{2}$ bilayer twisted approximately $3^{\circ}$ off the antiparallel alignment. Scanning tunneling spectroscopy reveals the presence of localized electronic states in the vicinity of the valence band onset. In particular, the onset of the valence band is observed to occur first in regions with a Bernal stacking in which S atoms are located on top of each other. In contrast, density-functional theory calculations on twisted bilayers which have been relaxed in vacuum predict the highest lying flat valence band to be localized in regions of AA' stacking. However, agreement with the experiment is recovered when the calculations are carried out on bilayers in which the atomic displacements from the unrelaxed positions have been reduced reflecting the influence of the substrate and finite temperature. This demonstrates the delicate interplay of atomic relaxations and the electronic structure of twisted bilayer materials.

cond-mat.mes-hall↗

arXiv2022

Ferroelectric switching at symmetry-broken interfaces by local control of dislocation networks

Semiconducting ferroelectric materials with low energy polarisation switching offer a platform for next-generation electronics such as ferroelectric field-effect transistors. Ferroelectric domains at symmetry-broken interfaces of transition metal dichalcogenide films provide an opportunity to combine the potential of semiconducting ferroelectrics with the design flexibility of two-dimensional material devices. Here, local control of ferroelectric domains in a marginally twisted WS2 bilayer is demonstrated with a scanning tunneling microscope at room temperature, and their observed reversible evolution understood using a string-like model of the domain wall network. We identify two characteristic regimes of domain evolution: (i) elastic bending of partial screw dislocations separating smaller domains with twin stacking and (ii) formation of perfect screw dislocations by merging pairs of primary domain walls. We also show that the latter act as the seeds for the reversible restoration of the inverted polarisation. These results open the possibility to achieve full control over atomically thin semiconducting ferroelectric domains using local electric fields, which is a critical step towards their technological use.

cond-mat.mes-hall↗

arXiv2021

Evidence for Higher order topology in Bi and Bi$_{0.92}$Sb$_{0.08}$

Higher order topological insulators (HOTIs) are a new class of topological materials which host protected states at the corners or hinges of a crystal. HOTIs provide an intriguing alternative platform for helical and chiral edge states and Majorana modes, but there are very few known materials in this class. Recent studies have proposed Bi as a potential HOTI, however, its topological classification is not yet well accepted. In this work, we show that the (110) facets of Bi and BiSb alloys can be used to unequivocally establish the topology of these systems. Bi and Bi$_{0.92}$Sb$_{0.08}$ (110) films were grown on silicon substrates using molecular beam epitaxy and studied by scanning tunneling spectroscopy. The surfaces manifest rectangular islands which show localized hinge states on three out of the four edges, consistent with the theory for the HOTI phase. This establishes Bi and Bi$_{0.92}$Sb$_{0.08}$ as HOTIs, and raises questions about the topological classification of the full family of Bi$_{x}$Sb$_{1-x}$ alloys.

cond-mat.mtrl-sci↗

arXiv2018

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

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 $T_c$ 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.

cond-mat.supr-con↗

arXiv2016

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

Mesoscopic point contacts between elemental metals and the topological 3D Dirac semimetal Cd$_3$As$_2$ have been recently shown to be superconducting with unconventional pairing while Cd$_3$As$_2$ itself does not superconduct. Here we show that the same superconducting phase at mesoscopic interfaces on Cd$_3$As$_2$ 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 (ABS) 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 high degree of spin-orbit coupling.

cond-mat.supr-con↗

arXiv2016

Comment on "Tip induced unconventional superconductivity on Weyl semimetal TaAs" [arXiv:1607.00513]

Recently, Wang $et$ $al.$ have reported the observation of "unconventional superconductivity" in the Weyl semimetal TaAs [arXiv:1607.00513]. The authors have written "\textit{A conductance plateau and sharp double dips are observed in the point contact spectra, indicating p-wave like unconventional superconductivity. Furthermore, the zero bias conductance peak in low temperature regime is detected, suggesting potentially the existence of Majorana zero modes. The experimentally observed tunneling spectra can be interpreted with a novel mirror-symmetry protected topological superconductor induced in TaAs, which can exhibit zero bias and double finite bias peaks, and double conductance dips in the measurements}." In this comment we show that for a superconducting point contact, the features like a zero-bias conductance peak, a plateau and single or multiple conductance dips might arise due to simple contact-heating related effects. Such features are routinely observed in point contacts involving a wide variety of superconductors when the experiments are not performed in the right regime of mesoscopic transport. We also show that the data presented by Wang $et$ $al.$ in a single transport regime of point contact do not confirm tip induced superconductivity (TISC). Even if it is assumed that Wang $et$ $al.$ achieved TISC on TaAs, all the spectra that they have reported show striking similarities with the type of spectra expected in thermal regime of transport. Such data cannot be used for extracting any spectroscopic information and based on such data any discussion on "p-wave" superconductivity or the emergence of Majorana modes should be considered invalid. This version (v2) also includes a brief discussion on the response of Wang $et$ $al.$ to the first version (v1) of this comment. Correct ballistic regime data on TaAs point contacts can be found in arXiv:1607.05131 (2016).

cond-mat.supr-con↗

arXiv2016

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

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 and magneto-transport properties and remarkably high surface spin polarization. Here we show that a unique mesoscopic superconducting phase with a critical temperature up to 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. The Andreev reflection spectra obtained from such point contacts are fitted well within a modified Blonder-Tinkham-Klapwijk (BTK) model with a superconducting energy gap up to 1.2 meV. The analysis within this model also reveals high transport spin polarization up to 60\% indicating a spin polarized supercurrent flowing through the point contacts on TaAs. Such point contacts also show a large anisotropic magnetoresistance (AMR) originating from the spin polarized current. Therefore, apart from the discovery of a novel mesoscopic superconducting phase and it's coexistence with a large spin polarization, our results also show that the point contacts on Weyl semimetals are potentially important for applications in spintronics.

cond-mat.supr-con↗

arXiv2016

Unexpected superconductivity at nanoscale junctions made on the topological crystalline insulator Pb$_{0.6}$Sn$_{0.4}$Te

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. Pb$_{0.6}$Sn$_{0.4}$Te 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 Pb$_{0.6}$Sn$_{0.4}$Te a novel superconducting phase is created locally in a confined region under the point-contact. This happens while 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 $T_c$ 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 new 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.

cond-mat.supr-con↗

arXiv2014

Unconventional Superconductivity at Mesoscopic Point-contacts on the 3-Dimensional Dirac Semi-metal Cd$_3$As$_2$

Since the three dimensional (3D) Dirac semi-metal Cd$_3$As$_2$ exists close to topological phase boundaries, in principle it should be possible to drive it into exotic new phases, like topological superconductors, by breaking certain symmetries. Here we show that the mesoscopic point-contacts between silver (Ag) and Cd$_3$As$_2$ exhibit superconductivity up to a critical temperature (onset) of 6 K while neither Cd$_3$As$_2$ 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 only in a quantum mechanically confined region under a point-contact between a Dirac semi-metal and a normal metal.

cond-mat.supr-con↗

arXiv2014

Direct evidence of strong local ferroelectric ordering in a thermoelectric semiconductor

It 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 $AgSbSe_{2}$.

cond-mat.mtrl-sci↗

arXiv2014

Observation of hysteretic phase-switching in silicon by piezoresponse force microscopy

We report the observation of $180^o$ phase switching on silicon wafers by piezo-response force microscopy (PFM). The switching is hysteretic and shows remarkable similarities with polarization switching in ferroelectrics. 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.

cond-mat.mes-hall↗