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J. Nayak

Publications and source records attributed to J. Nayak.

10 recordsLinked to original sources
arXiv2022

Weak antilocalization effect and triply degenerate state in Cu-doped CaAuAs

The effect of 50\% Cu doping at the Au site in the topological Dirac semimetal CaAuAs is investigated through electronic band structure calculations, electrical resistivity, and magnetotransport measurements. Electronic structure calculations a suggest broken-symmetry-driven topological phase transition from the Dirac to triple-point state in CaAuAs via alloy engineering. The electrical resistivity of both the CaAuAs and CaAu$_{0.5}$Cu$_{0.5}$As compounds shows metallic behavior. Nonsaturating quasilinear magnetoresistance (MR) behavior is observed in CaAuAs. On the other hand, MR of the doped compound shows a pronounced cusplike feature in the low-field regime. Such behavior of MR in CaAu$_{0.5}$Cu$_{0.5}$As is attributed to the weak antilocalization (WAL) effect. The WAL effect is analyzed using different theoretical models, including the semiclassical $\sim\sqrt{B}$ one which accounts for the three-dimensional WAL and modified Hikami-Larkin-Nagaoka model. Strong WAL effect is also observed in the longitudinal MR, which is well described by the generalized Altshuler-Aronov model. Our study suggests that the WAL effect originates from weak disorder and the spin-orbit coupled bulk state. Interestingly, we have also observed the signature of chiral anomaly in longitudinal MR, when both current and field are applied along the $c$ axis. The Hall resistivity measurements indicate that the charge conduction mechanism in these compounds is dominated by the holes with a concentration $\sim$10$^{20}$ cm$^{-3}$ and mobility $\sim 10^2$ cm$^2$ V$^{-1}$ S$^{-1}$.

cond-mat.str-el↗
arXiv2022

Large nonsaturating magnetoresistance, weak anti-localization and non-trivial topological states in SrAl$_2$Si$_2$

We explore the electronic and topological properties of single crystal SrAl$_2$Si$_2$ using magnetotransport experiments in conjunction with first-principle calculations. We find that the temperature-dependent resistivity shows a pronounced peak near 50 K. We observe several remarkable features at low temperatures, such as large non-saturating magnetoresistance, Shubnikov-de Haas oscillations and cusp-like magneto-conductivity. The maximum value of magnetoresistance turns out to be 459\% at 2 K and 12 T. The analysis of the cusp-like feature in magneto-conductivity indicates a clear signature of weak anti-localization. Our Hall resistivity measurements confirm the presence of two types of charge carriers in SrAl$_2$Si$_2$, with low carrier density.

cond-mat.str-el↗
arXiv2020

Three interaction energy scales in single-layer high-T$_C$ cuprate HgBa$_2$CuO$_{4+δ}$

The lamellar cuprate superconductors exhibit the highest ambient-pressure superconducting transition temperatures (T$_C$) and, after more than three decades of extraordinary research activity, continue to pose formidable scientific challenges. A major experimental obstacle has been to distinguish universal phenomena from materials- or technique-dependent ones. Angle-resolved photoemission spectroscopy (ARPES) measures momentum-dependent single-particle electronic excitations and has been invaluable in the endeavor to determine the anisotropic momentum-space properties of the cuprates. HgBa$_2$CuO$_{4+δ}$ (Hg1201) is a single-CuO$_2$-layer cuprate with a particularly high optimal T$_C$ and a simple crystal structure; yet there exists little information from ARPES about the electronic properties of this model system. Here we present an ARPES study of doping-, temperature-, and momentum-dependent systematics of near-nodal dispersion anomalies in Hg1201. The data reveal a hierarchy of three distinct energy scales -a sub-gap low-energy kink, an intermediate-energy kink near 55 meV, and a peak-dip-hump structure. The first two features are attributed to the coupling of electrons to Ba-derived optical phonons and in-plane bond-stretching phonons, respectively. The nodal peak-dip-hump structure appears to have a common doping-dependence in several single-layer cuprates, and is interpreted as a manifestation of pseudogap physics at the node. These results establish several universal phenomena, both in terms of connecting multiple experimental techniques for a single material, and in terms of connecting comparable spectral features in multiple structurally similar cuprates.

cond-mat.supr-con↗
arXiv2019

Evidence of hot and cold spots on the Fermi surface of LiFeAs

Angle-resolved photoemission spectroscopy (ARPES) is used to study the energy and momentum dependence of the inelastic scattering rates and the mass renormalization of charge carriers in LiFeAs at several high symmetry points in the Brillouin zone. A strong and linear-in-energy scattering rate is observed for sections of the Fermi surface having predominantly Fe $3d_{xy/yz}$ orbital character on the inner hole and on electron pockets. We assign them to hot spots with marginal Fermi liquid character inducing high antiferromagnetic and pairing susceptibilities. The outer hole pocket, with Fe $3d_{xy}$ orbital character, has a reduced but still linear in energy scattering rate. Finally, we assign sections on the middle hole pockets with Fe $3d_{xz,yz}$ orbital character and on the electron pockets with Fe $3d_{xy}$ orbital character to cold spots because there we observe a quadratic-in-energy scattering rate with Fermi-liquid behavior. These cold spots prevail the transport properties. Our results indicate a strong $\it{momentum}$ dependence of the scattering rates. We also have indications that the scattering rates in correlated systems are fundamentally different from those in non-correlated materials because in the former the Pauli principle is not operative. We compare our results for the scattering rates with combined density functional plus dynamical mean-field theory calculations. The work provides a generic microscopic understanding of macroscopic properties of multiorbital unconventional superconductors.

cond-mat.supr-con↗
arXiv2016

Experimental evidence for the importance of Hund's exchange interaction for the incoherence of the charge carriers in iron-based superconductors

Angle-resolved photoemission spectroscopy (ARPES) is used to study the scattering rates of charge carriers from the hole pockets near Gamma in the iron-based high-Tc hole doped superconductors KxBa1-xFe2As2 x=0.4 and KxEu1-xFe2As2 x=0.55$ and the electron doped compound Ba(Fe1-xCox)2As2 x=0.075. The scattering rate for any given band is found to depend linearly on energy, indicating a non-Fermi liquid regime. The scattering rates in the hole-doped compound are considerably larger than those in the electron-doped compounds. In the hole-doped systems the scattering rate of the charge carriers of the inner hole pocket is about three times bigger than the binding energy indicating that the spectral weight is heavily incoherent. The strength of the scattering rates and the difference between electron and hole doped compounds signals the importance of Hund's exchange coupling for correlation effects in these iron-based high-Tc superconductors. The experimental results are in qualitative agreement with theoretical calculations in the framework of combined density functional dynamical mean-field theory.

cond-mat.supr-con↗
arXiv2014

Inverse photoemission spectroscopic studies on phase separated La$_{0.2}$Sr$_{0.8}$MnO$_{3}$

We have studied the temperature evolution of the inverse photoemission spectra of phase separated La$_{0.2}$Sr$_{0.8}$MnO$_{3}$. To identify the features in the room temperature experimental spectra, band structure calculations using Korringa-Kohn-Rostoker Green's function method were carried out. We find that the features generated by local moment disorder calculations give a better match with the experimental spectrum. In the insulating phase, we observed unusually an increased intensity at around the Fermi level. This puzzling behaviour is attributed to the shift in the chemical potential towards the conduction band. The present results clearly show the importance of unoccupied electronic states in better understanding of the phase separated systems.

cond-mat.str-el↗
arXiv2014

Gamma/hadron segregation for a ground based imaging atmospheric Cherenkov telescope using machine learning methods: Random Forest leads

A detailed case study of $γ$-hadron segregation for a ground based atmospheric Cherenkov telescope is presented. We have evaluated and compared various supervised machine learning methods such as the Random Forest method, Artificial Neural Network, Linear Discriminant method, Naive Bayes Classifiers,Support Vector Machines as well as the conventional dynamic supercut method by simulating triggering events with the Monte Carlo method and applied the results to a Cherenkov telescope. It is demonstrated that the Random Forest method is the most sensitive machine learning method for $γ$-hadron segregation.

astro-ph.IM↗
arXiv2014

Validation of a new background discrimination method for the TACTIC TeV $γ$-ray telescope with Markarian 421 data

This paper describes the validation of a new background discrimination method based on Random Forest technique by re-analysing the Markarian 421 (Mrk 421) observations performed by the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera) gamma-ray telescope. The Random Forest technique is a flexible multivariate method which combines Bagging and Random Split Selection to construct a large collection of decision trees and then combines them to construct a common classifier. Markarian 421 in a high state was observed by TACTIC during December 07, 2005 - April 30, 2006 for 202 h. Previous analysis of this data led to a detection of flaring activity from the source at Energy $ >$ 1 TeV. Within this data set, a spell of 97 h revealed strong detection of a gamma-ray signal with daily flux of > 1 Crab unit on several days. Here we re-analyze this spell as well as the data from the entire observation period with the Random Forest method. Application of this method led to an improvement in the signal detection strength by $\sim 26\%$ along with a $\sim 18\%$ increase in detected $γ$ rays compared to the conventional Dynamic Supercuts method. The resultant differential spectrum obtained is represented by a power law with an exponential cut off $Γ= -2.51 \pm 0.10$ and $E_{0} = 4.71 \pm 2.20$ TeV. Such a spectrum is consistent with previously reported results and justifies the use of Random Forest method for analyzing data from atmospheric Cherenkov telescopes.

astro-ph.HE↗
arXiv2006

Working Group Report: Heavy-Ion Physics and Quark-Gluon Plasma

This is the report of Heavy Ion Physics and Quark-Gluon Plasma at WHEPP-09 which was part of Working Group-4. Discussion and work on some aspects of Quark-Gluon Plasma believed to have created in heavy-ion collisions and in early universe are reported.

hep-ph↗