arXiv subjects

S. Tewari

Publications and source records attributed to S. Tewari.

11 recordsLinked to original sources

arXiv2022

Dynamic bonding influenced by the proximity of adatoms to one-atom high step edges

Low-temperature scanning tunneling microscopy is used here to study dynamic bonding of gold atoms on surfaces under low coordination conditions. In the experiments, using an atomically-sharp gold tip, a gold adatom is deposited onto a gold surface with atomic precision either on the first hollow site near a step edge, or far away from it. Classical molecular dynamics simulations at 4.2 K and density functional theory calculations serve to elucidate the difference in the bonding behavior between these two different placements, while also providing information on the crystalline classification of the STM tips based on their experimental performance.

cond-mat.mtrl-sci↗

arXiv2018

Chiral topological phases in optical lattices without synthetic fields

Synthetic fields applied to ultracold quantum gases can realize topological phases that transcend conventional Bose and Fermi-liquid paradigms. Raman laser beams in particular are under scrutiny as a route to create synthetic fields in neutral gases to mimic ordinary magnetic and electric fields acting on charged matter. Yet external laser beams can impose heating and losses that make cooling into many-body topological phases challenging. We propose that atomic or molecular dipoles placed in optical lattices can realize a topological phase without synthetic fields by placing them in certain frustrated lattices. We use numerical modeling on a specific example to show that the interactions between dipolar fermions placed in a kagome optical lattice spontaneously break time reversal symmetry to lead to a topological Mott insulator, a chiral topological phase generated entirely by interactions. We estimate realistic entropy and trapping parameters to argue that this intriguing phase of matter can be probed with quantum gases using a combination of recently implemented technologies.

cond-mat.str-el↗

arXiv2015

Robust quantum coherence above the Fermi sea

In this paper we present an experiment where we measured the quantum coherence of a quasiparticle injected at a well-defined energy above the Fermi sea into the edge states of the integer quantum Hall regime. Electrons are introduced in an electronic Mach-Zehnder interferometer after passing through a quantum dot that plays the role of an energy filter. Measurements show that above a threshold injection energy, the visibility of the quantum interferences is almost independent of the energy. This is true even for high energies, up to 130~$μ$eV, well above the thermal energy of the measured sample. This result is in strong contradiction with our theoretical predictions, which instead predict a continuous decrease of the interference visibility with increasing energy. This experiment raises serious questions concerning the understanding of excitations in the integer quantum Hall regime.

cond-mat.mes-hall↗

arXiv2015

Majorana fermions in ferromagnetic chains on the surface of bulk spin-orbit coupled $s$-wave superconductors

Majorana fermion (MF) excitations in solid state system have non-Abelian statistics which is essential for topological quantum computation. Previous proposals to realize MF, however, generally requires fine-tuning of parameters. Here we explore a platform which avoids the fine-tuning problem, namely a ferromagnetic chain deposited on the surface of a spin-orbit coupled $s$-wave superconductor. We show that it generically supports zero-energy topological MF excitations near the two ends of the chain with minimal fine-tuning. Depending on the strength of the ferromagnetic moment in the chain, the number of MFs at each end, $n$, can be either one or two, and should be revealed by a robust zero-bias peak (ZBP) of height $2ne^2/h$ in scanning tunneling microscopy (STM) measurements which would show strong (weak) signals at the ends (middle) of the chain. The role of an approximate chiral symmetry which gives an integer topological invariant to the system is discussed.

cond-mat.mes-hall↗

arXiv2015

Robust local and non-local transport in the Topological Kondo Insulator SmB$_{6}$ in the presence of high magnetic field

SmB$_6$ has been predicted to be a Kondo Topological Insulator with topologically protected conducting surface states. We have studied quantitatively the electrical transport through surface states in high quality single crystals of SmB$_6$. We observe a large non-local surface signal at temperatures lower than the bulk Kondo gap scale. Measurements and finite element simulations allow us to distinguish unambiguously between the contributions from different transport channels. In contrast to general expectations, the electrical transport properties of the surface channels was found to be insensitive to high magnetic fields. Local and non-local magnetoresistance measurements allowed us to identify definite signatures of helical spin states and strong inter-band scattering at the surface.

cond-mat.mes-hall↗

arXiv2014

Hidden Symmetry Decoupling of Majorana Fermions

Multiple zero-energy Majorana fermions (MFs) with spatially overlapping wave functions can survive only if their splitting is prevented by an underlying symmetry. Here we show that, in quasi-one-dimensional (Q1D) time reversal invariant topological superconductors (class DIII), a realistic model for superconducting lithium molybdenum purple bronze and certain families of organic superconductors, multiple Majorana-Kramers pairs with strongly overlapping wave functions persist at zero energy even in the absence of an easily identifiable symmetry. We find that similar results hold in the case of Q1D semiconductor-superconductor heterostructures (class D) with transverse hopping t_{perp} much smaller than longitudinal hopping t_x. Our results, explained in terms of special properties of the Hamiltonian and wave functions, underscore the importance of hidden accidental symmetries in topological superconductors.

cond-mat.mes-hall↗

arXiv2011

Controlling non-Abelian statistics of Majorana fermions on Majorana dimer lattices

Under appropriate external conditions a semiconductor nanowire in proximity to an s-wave superconductor can be in a topological superconducting (TS) phase. This phase supports localized zero-energy Majorana fermions at the ends of the wire. However, the non-Abelian exchange statistics of Majorana fermions is difficult verify because of the one-dimensional topology of such wires. In this paper we propose a scheme to transport Majorana fermions between the ends of different wires using tunneling, which is shown to be controllable by gate voltages. Such tunneling-generated hops of Majorana fermions can be used to exchange the Majorana fermions. The exchange process thus obtained is described by a non-Abelian braid operator that is uniquely determined by the well-controlled microscopic tunneling parameters.

cond-mat.mes-hall↗

arXiv2008

Boson Hubbard model with weakly coupled Fermions

Using an imaginary-time path integral approach, we develop the perturbation theory suited to the boson Hubbard model, and apply it to calculate the effects of a dilute gas of spin-polarized fermions weakly interacting with the bosons. The full theory captures both the static and the dynamic effects of the fermions on the generic superfluid-insulator phase diagram. We find that, in a homogenous system described by a single-band boson Hubbard Hamiltonian, the intrinsic perturbative effect of the fermions is to generically suppress the insulating lobes and to enhance the superfluid phase.

cond-mat.str-el↗

arXiv2005

Blue Quantum Fog: Chiral Condensation in Quantum Helimagnets

It is shown that a condensation transition involving a chiral order parameter can occur in itinerant helimagnets, in analogy to the transition between the isotropic phase and the phase known as blue fog or blue phase III in cholesteric liquid crystals. It is proposed that such a transition is the explanation for recent neutron scattering results in MnSi. Predictions are made that will allow to experimentally test this proposal.

cond-mat.str-el↗

arXiv2003

Plant defense multigene families: II Evolution of coding sequence and differential expression of PR10 genes in Pisum

While it is not possible to directly the observe evolution of multigene families, the best alternative is to compare orthologous family members among several closely-related species with varying degrees of reproductive isolation. Using RT-PCR we show that in pea (Pisum sativum) each member of the pathogenesis-related PR10 family has a distinct pattern of expression in response to the fungus Fusarium solani, and in treatment with salicylic acid, chitosan and abcisic acid. Sequencing reveals that PR10.1, PR10.2 and PR10.3 exist in P. humile, P. elatius and P. fulvum, except that no PR10.2 orthologue was identified in P. elatius. PR10.1, PR10.2 and PR10.3 appear to have diverged from a single gene in the common Pisum ancestor. For the recently diverged PR10.1 and PR10.2, the timing of fungal-induced expression differs greatly among species. For example, PR10.1 was strongly induced in P. sativum by F. solani within 8 hours postinoculation (h.p.i.), whereas little PR10.1 expression was seen in pea's closest relative, P. humile, and in the more distantly-related P. elatius. In P. fulvum, expression did not peak until 48 h.p.i. Expression of the more ancient PR10.4 and PR10.5 genes is more tightly conserved among Pisum species. These data indicate that expression, as well as sequence, can evolve rapidly. We hypothesize that changes in differential expression of multigene family members could provide a source of phenotypic diversity in populations, which may be of particular importance to plant/pathogen coevolution.

q-bio.PE↗

arXiv1994

Nesting Mechanism for d-symmetry Superconductors

A nested Fermi surface with nearly parallel orbit segments is found to yield a singlet d-wave superconducting state at high temperatures for a restricted range of the on-site Coulomb repulsion that avoids the competing spin density wave instability. The computed superconducting transition temperature drops dramatically as the nesting vector is decreased, in accord with recent photo-emission data on the Bi2212 and Bi2201 cuprates.

cond-mat↗