arXiv subjects

P. V. Sreenivasa Reddy

Publications and source records attributed to P. V. Sreenivasa Reddy.

10 recordsLinked to original sources

arXiv2026

Strong-coupling anisotropic superconductivity in hexagonal HfRuAs from anisotropic Migdal-Eliashberg theory

We present a comprehensive theoretical investigation of the superconducting (SC) properties of hexagonal HfRuAs ($h$-HfRuAs) by solving anisotropic Migdal--Eliashberg (ME) equations with the inputs from \textit{ab initio} calculations of electronic structure, phonon dispersion and electron phonon coupling matrix elements. The calculated Eliashberg spectral function reveals strong electron--phonon coupling (EPC) with a constant $λ\approx 1.56$, dominated by low-frequency phonon modes associated primarily with Hf and Ru vibrations. The SC state is characterized by a single anisotropic gap with overall $s$-wave symmetry, as evidenced by the fully gapped quasiparticle density of states. The momentum-resolved EPC and SC gap exhibit pronounced anisotropy across different Fermi surface sheets, with the largest variations occurring on the hole-like bands. The SC gap is centered around $Δ\approx 2.9$ meV with a spread of $\sim 0.8$ meV, indicating significant multiband anisotropy. The resulting gap ratio $2Δ(0)/k_B T_c \approx 4.2$ exceeds the BCS weak-coupling limit, establishing $h$-HfRuAs as a strong-coupling superconductor. The calculated transition temperature, $T_c$, agrees in the order of magnitude with experiments. Overall, our results identify $h$-HfRuAs as a phonon-mediated, strongly coupled anisotropic superconductor and provide detailed insights into the role of momentum-dependent electron--phonon interactions in determining its SC properties.

cond-mat.supr-con↗

arXiv2026

Anisotropic magnetoresistance and magnetic field-tunable Weyl nodes in Weyl metal SrRuO$_{3}$ thin films

Weyl semimetals are a unique class of topological materials, possessing Fermi-arc surface states and exhibiting the chiral anomaly effect. The chiral anomaly refers to non-equilibrium charge transfer within a Weyl-node pair of opposite chirality under the condition of aligned electric and magnetic fields ($\bf{E} \parallel \bf{B}$), leading to non-conserved chiral charges and thus enhanced electrical conductivity. In experiments, such an enhanced conductivity due to the chiral anomaly manifests as a negative longitudinal magnetoresistance (MR) when the external field $\bf{H}$ is applied along the bias current direction $\bf{I}$. In this work, we present rigorous $ϕ$- and $α$-dependent magnetotransport measurements to investigate such a negative longitudinal MR due to the chiral anomaly in a sunbeam-shaped device fabricated from an untwinned Weyl metal SrRuO$_{3}$ (SRO) thin film. Here, $ϕ$($α$) represents the angle between $\bf{I}$ and the in-plane $\bf{H}$(SRO monoclinic [001]$_{\rm o}$). Unusual $ϕ$ dependences of in-plane MR and Hall effects were uncovered at low temperatures, accompanied by the emergence of the fourfold-symmetric component in the in-plane MR. These results indicate that the chiral anomaly and resistivity anisotropy in SRO play important roles. In particular, the dramatic variation of Weyl nodes near the Fermi level through magnetic field manipulation of the magnetization orientation, as revealed by band structure calculations, is consistent with the observed in-plane MR and Hall effect.

cond-mat.other↗

arXiv2025

Coexistent topological and chiral phonons in chiral RhGe: An ab initio study

The CoSi-family of materials (CoSi, CoGe, RhSi and RhGe) forms a cubic chiral structure and hosts unconventional multifold chiral fermions, such as spin-1 and spin-3/2 fermions, leading to intriguing phenomena like long Fermi arc surface states and exotic transport properties. Recent interest on the phonon behavior in chiral materials is growing due to their unique characteristics, including topological phonons, protected surface states and the chiral phonons with non-zero angular momentums. In this study, we explore the topological and chiral phonon behavior in RhGe, using first-principles density functional theory calculations as well as the symmetry and topological analysis. In particular, we uncover six spin-1 triply degenerate nodal points at the $Γ$ point and six charge-2 double Weyl points at the R point in the Brillouin zone (BZ). Interestingly, these topological features are identical to that in the electronic band structure without the electron spin-orbit coupling, of the same material. We expect that this finding not only applies to the CoSi family but also is universal. Secondly, we find that chiral crystal RhGe hosts chiral phonon modes with a phonon angular momentum (PAM) and an associated phonon magnetic moment (PMM), everywhere in the BZ except at high symmetry points such as $Γ$, R, X and M. The PAM and PMM are large along the chiral rotation axis and also in the vicinity of the topological nodes. Our study also reveals that all the topological phonon modes are chiral. However, the reverse is not always true. Among other things, our finding of the coexistence of topological and chiral phonon modes in chiral RhGe not only deepens our understanding of the phonon behavior in the CoSi-family but also opens new pathways for developing advanced materials and devices.

cond-mat.mtrl-sci↗

arXiv2024

Nonlinear and nonreciprocal transport effects in untwinned thin films of ferromagnetic Weyl metal SrRuO$_3$

The identification of distinct charge transport features, deriving from nontrivial bulk band and surface states, has been a challenging subject in the field of topological systems. In topological Dirac and Weyl semimetals, nontrivial conical bands with Fermi-arc surface states give rise to negative longitudinal magnetoresistance due to chiral anomaly effect and unusual thickness dependent quantum oscillation from Weyl-orbit effect, which were demonstrated recently in experiments. In this work, we report the experimental observations of large nonlinear and nonreciprocal transport effects for both longitudinal and transverse channels in an untwinned Weyl metal of SrRuO$_3$ thin film grown on a SrTiO$_{3}$ substrate. From rigorous measurements with bias current applied along various directions with respect to the crystalline principal axes, the magnitude of nonlinear Hall signals from the transverse channel exhibits a simple sin$α$ dependence at low temperatures, where $α$ is the angle between bias current direction and orthorhombic [001]$_{\rm o}$, reaching a maximum when current is along orthorhombic [1-10]$_{\rm o}$. On the contrary, the magnitude of nonlinear and nonreciprocal signals in the longitudinal channel attains a maximum for bias current along [001]$_{\rm o}$, and it vanishes for bias current along [1-10]$_{\rm o}$. The observed $α$-dependent nonlinear and nonreciprocal signals in longitudinal and transverse channels reveal a magnetic Weyl phase with an effective Berry curvature dipole along [1-10]$_{\rm o}$ from surface states, accompanied by 1D chiral edge modes along [001]$_{\rm o}$.

cond-mat.mes-hall↗

arXiv2023

Evidence for Unconventional Superconductivity and Nontrivial Topology in PdTe

PdTe is a superconductor with Tc ~4.25 K. Recently, evidence for bulk-nodal and surface-nodeless gap features has been reported in PdTe [Yang et al., Phys. Rev. Lett. 130, 046402 (2023)]. Here, we investigate the physical properties of PdTe in both the normal and superconducting states via specific heat and magnetic torque measurements and first-principles calculations. Below Tc, the electronic specific heat initially decreases in T3 behavior (1.5 K < T < Tc) then exponentially decays. Using the two-band model, the superconducting specific heat can be well described with two energy gaps: one is 0.372 meV and another 1.93 meV. The calculated bulk band structure consists of two electron bands (α and \b{eta}) and two hole bands (γ and η) at the Fermi level. Experimental detection of the de Haas-van Alphen (dHvA) oscillations allows us to identify four frequencies (Fα = 65 T, F\b{eta} = 658 T, Fγ = 1154 T, and Fη = 1867 T for H // a), consistent with theoretical predictions. Nontrivial α and \b{eta} bands are further identified via both calculations and the angle dependence of the dHvA oscillations. Our results suggest that PdTe is a candidate for unconventional superconductivity.

cond-mat.supr-con↗

arXiv2023

Observation of 2D Weyl Fermion States in Epitaxial Bismuthene

A two-dimensional (2D) Weyl semimetal featuring a spin-polarized linear band dispersion and a nodal Fermi surface is a new topological phase of matter. It is a solid-state realization of Weyl fermions in an intrinsic 2D system. The nontrivial topology of 2D Weyl cones guarantees the existence of a new form of topologically protected boundary states, Fermi string edge states. In this work, we report the realization of a 2D Weyl semimetal in monolayer-thick epitaxial bismuthene grown on SnS(Se) substrate. The intrinsic band gap of bismuthene is eliminated by the space-inversion-symmetry-breaking substrate perturbations, resulting in a gapless spin-polarized Weyl band dispersion. The linear dispersion and spin polarization of the Weyl fermion states are observed in our spin and angle-resolved photoemission measurements. In addition, the scanning tunneling microscopy/spectroscopy reveals a pronounced local density of states at the edge, suggesting the existence of Fermi string edge states. These results open the door for the experimental exploration of the exotic properties of Weyl fermion states in reduced dimensions.

cond-mat.mtrl-sci↗

arXiv2021

Observation of Unpinned Two-Dimensional Dirac States in Antimony Single Layers with Phosphorene Structure

The discovery of graphene has stimulated enormous interest in two-dimensional (2D) electron gas with linear band structure. 2D Dirac materials possess many intriguing physical properties such as high carrier mobility and zero-energy Landau level thanks to the relativistic dispersion and chiral spin/pseudospin texture. 2D Dirac states discovered so far are exclusively pinned at high-symmetry points of the Brillouin zone, for example, surface Dirac states at $\overlineΓ$ in topological insulators Bi$_2$Se(Te)$_3$ and Dirac cones at $K$ and $K'$ in graphene. In this work, we report the realization of 2D Dirac states at generic $k$-points in antimony atomic layers with phosphorene structure ($i.e.$ $α$-antimonene). The unpinned nature enables versatile ways to control the locations of the Dirac points in momentum space. In addition, dispersions around the unpinned Dirac points exhibit intrinsically anisotropic behaviors due to the reduced symmetry of generic momentum points. These properties make the $α$-antimonene films a promising platform for exploring interesting physics in unpinned 2D Dirac fermions that are distinct from the conventional Dirac states in graphene.

cond-mat.mes-hall↗

arXiv2021

Observation of Symmetry-Protected Dirac States in Nonsymmorphic $α$-Antimonene

Two-dimensional (2D) Dirac states with linear band dispersion have attracted enormous interest since the discovery of graphene. However, to date, 2D Dirac semimetals are still very rare due to the fact that 2D Dirac states are generally fragile against perturbations such as spin-orbit couplings. Nonsymmorphic crystal symmetries can enforce the formation of Dirac nodes, providing a new route to establishing symmetry-protected Dirac states in 2D materials. Here we report the symmetry-protected Dirac states in nonsymmorphic alpha-antimonene. The antimonene was synthesized by the method of molecular beam epitaxy. Two Dirac cones with large anisotropy were observed by angle-resolved photoemission spectroscopy. The Dirac state in alpha-antimonene is of spin-orbit type in contrast to the spinless Dirac states in graphene. The result extends the 'graphene' physics into a new family of 2D materials where spin-orbit coupling is present.

cond-mat.mes-hall↗

arXiv2019

Inducing half metallicity with alloying in Heusler Compound CoFeMnSb

First principles studies were performed in order to find out the possibility of inducing half-metallicity in Heusler Compound CoFeMnSb, by means of alloying it with 3d-transition metal elements. Proper alloying element is selected through the calculations of formation energies. These calculations were tested with different concentrations of alloying elements at different atomic sites. Among the selected transition metal elements Sc and Ti are proposed to be excellent alloying elements particularly at Mn site. By using these alloying elements complete half metallic behaviour is obtained in CoFeMn0.25Sc0.75Sb, CoFeMn0.75Ti0.25Sb, CoFeMn0.625Ti0.375Sb, CoFeMn0.50Ti0.50Sb, CoFeMn0.25Ti0.75Sb and CoFeTiSb alloys. Shifting of Co-Fe d-states towards lower energy region leads to zero density of states at Fermi level for the spin minority channel. Alloying effects on the electronic structure and magnetization are discussed in details. Thermodynamical stability of these new alloys are major part of this study. The Curie temperatures of CoFeMn0.25Sc0.75Sb and CoFeMn0.75Ti0.25Sb were found to be 324.5 K and 682 K; respectively, showing good candidature for spintronics applications. For understanding the bonding nature of constituent atom of CoFeMnSb, crystal orbital Hamiltonian populations have been analysed.

cond-mat.mtrl-sci↗

arXiv2016

Electronic topological transitions in Nb$_3$X (X = Al, Ga, In, Ge and Sn) under compression investigated by first principles calculations

First principles electronic structure calculations of A-15 type Nb$_3$X (X = Al, Ga, In, Ge and Sn) compounds are performed at ambient and high pressures. Mechanical stability is confirmed in all the compounds both at ambient as well as under compression from the calculated elastic constants. We have observed four holes and two electron Fermi surfaces (FS) for all the compounds studied and FS nesting feature is observed at M and along X - $Γ$ in all the compounds. A continuous change in the FS topology is observed under pressure in all the compounds which is also reflected in the calculated elastic constants and density of states under pressure indicating the Electronic topological transitions (ETT). The ETT observed at around 21.5 GPa, 17.5 GPa in Nb$_3$Al and Nb$_3$Ga are in good agreement with the anomalies observed by the experiments.

cond-mat.mtrl-sci↗