arXiv subjects

Dipanjan Chaudhuri

Publications and source records attributed to Dipanjan Chaudhuri.

26 records · Page 2Linked to original sources

arXiv2020

Impact of the lattice on magnetic properties and possible spin nematicity in the S=1 triangular antiferromagnet NiGa$_2$S$_4$

NiGa$_2$S$_4$ is a triangular lattice S=1 system with strong two-dimensionality of the lattice, actively discussed as a candidate to host spin-nematic order brought about by strong quadrupole coupling. Using Raman scattering spectroscopy we identify a phonon of E$_g$ symmetry which can modulate magnetic exchange $J_1$ and produce quadrupole coupling. Additionally, our Raman scattering results demonstrate a loss of local inversion symmetry on cooling, which we associate with sulfur vacancies. This will lead to disordered Dzyaloshinskii-Moriya interactions, which can prevent long range magnetic order. Using magnetic Raman scattering response we identify 160~K as a temperature of an upturn of magnetic correlations. The temperature below 160~K, but above 50~K where antiferromagnetic magnetic start to increase, is a candidate for spin-nematic regime.

cond-mat.str-el↗

arXiv2020

Near-surface ocean kinetic energy spectra and small scale intermittency from ship data in the Bay of Bengal

Horizontal currents in the Bay of Bengal were measured on eight cruises covering a total of 8600 $km$ using a 300 $kHz$ Acoustic Doppler Current Profiler (ADCP). The cruises are distributed over multiple seasons and regions of the Bay. Horizontal wavenumber spectra of these currents over depths of 12--54 m and wavelengths from 2--400 km were decomposed into vortical and divergent components assuming isotropy. An average of across and along track spectra over all cruises shows that the spectral slope of horizontal kinetic energy for wavelengths of 10--80 $km$ scales with an exponent of $-1.7 \pm 0.05$, which transitions to steeper scaling for wavelengths above 80 $km$. The rotational component is significantly larger than the divergent component at scales greater than 80 $km$, while the two are comparable for smaller wavelengths. The measurements show a fair amount of variability and spectral levels vary between cruises by about a factor of 5 over 10--100 $km$. Velocity differences over 10--80 $km$ show probability density functions and structure functions with stretched exponential behavior and anomalous scaling. Comparisons with the Garrett-Munk internal wave spectrum indicate that inertia-gravity waves account for only a small fraction of the kinetic energy except at the smallest scales. These constraints suggest that the near-surface flow in the Bay is primarily balanced and follows a forward enstrophy transfer quasigeostrophic regime for wavelengths greater than approximately 80 km, with a larger role for unbalanced rotating stratified turbulence and internal waves at smaller scales.

physics.ao-ph↗

arXiv2018

Investigation of Thermodynamic Properties of Cu(NH3)4SO4.H2O, a Heisenberg Spin Chain Compound

Detailed experimental investigations of thermal and magnetic properties are presented for Cu(NH3)4SO4.H2O, an ideal uniform Heisenberg spin half chain compound. A comparison of these properties with relevant spin models is also presented. The temperature dependent magnetic susceptibility and specific heat data has been compared with the exact solution for uniform Heisenberg chain model derived by means of Bethe ansatz technique. Field dependent isothermal magnetization curves are simulated by Quantum Monte Carlo technique and compared with the corresponding experimental ones. Specific heat as a function of magnetic field (up to 7T) and temperature (down to 2K) is reported. Subsequently, the data are compared with the corresponding theoretical curves for the infinite Heisenberg spin half chain model with J=6K. Moreover, internal energy and entropy are calculated by analyzing the experimental specific heat data. Magnetic field and temperature dependent behavior of entropy and internal energy are in good agreement with the theoretical predictions.

cond-mat.str-el↗

arXiv2018

Electric field modulated topological magnetoelectric effect in Bi$_2$Se$_3$

Topological insulators have been predicted to exhibit a variety of interesting phenomena including a quantized magnetoelectric response and novel spintronics effects due to spin textures on their surfaces. However, experimental observation of these phenomena has proved difficult due to the finite bulk carrier density which may overwhelm the intrinsic topological responses that are expressed at the surface. Here, we demonstrate a novel ionic gel gating technique to tune the chemical potential of Bi$_{2}$Se$_{3}$ thin films while simultaneously performing THz spectroscopy. We can tune the carrier concentration by an order of magnitude and shift the Fermi energy, E$_{F} $ to as low as $\simeq$ 10 meV above the Dirac point. At high bias voltage and magnetic field, we observe a quantized Faraday angle consistent with the topological magnetoelectric effect that can be tuned by ionic gel gating through a number of plateau states

cond-mat.mes-hall↗

arXiv2017

Dielectric anomalies and interactions in the 3D quadratic band touching Luttinger semimetal Pr$_2$Ir$_2$O$_7$

Dirac and Weyl semimetals with linearly crossing bands are the focus of much recent interest in condensed matter physics. Although they host fascinating phenomena, their physics can be understood in terms of weakly interacting electrons. In contrast, more than 40 years ago, Abrikosov pointed out that quadratic band touchings are generically strongly interacting. We have performed terahertz spectroscopy on films of the conducting pyrochlore Pr$_2$Ir$_2$O$_7$, which has been shown to host a quadratic band touching. A dielectric constant as large as $\tilde{\varepsilon }/ε_0 \sim 180 $ is observed at low temperatures. In such systems the dielectric constant is a measure of the relative scale of interactions, which are therefore in our material almost two orders of magnitude larger than the kinetic energy. Despite this, the scattering rate exhibits a $T^2$ dependence, which shows that for finite doping a Fermi liquid state survives, however with a scattering rate close to the maximal value allowed.

cond-mat.str-el↗

arXiv2017

An optical investigation of the strong spin-orbital coupled magnetic semimetal YbMnBi$_2$

Strong spin-orbit coupling (SOC) can result in ground states with non-trivial topological properties. The situation is even richer in magnetic systems where the magnetic ordering can potentially have strong influence over the electronic band structure. The class of AMnBi$_2$ (A = Sr, Ca) compounds are important in this context as they are known to host massive Dirac fermions with strongly anisotropic dispersion that is believed to be due to the interplay between strong SOC and magnetic degrees of freedom. We report the optical conductivity of YbMnBi$_2$, a newly discovered member of this family and a proposed Weyl semi-metal (WSM) candidate with broken time reversal symmetry. Together with density functional theory (DFT) band structure calculations, we show how the complete conductivity can be interpreted as the sum of a intra-band Drude response and inter-band transitions. We argue that the canting of the magnetic moments that has been proposed to be essential for the realization of the WSM in an otherwise antiferromagnetically ordered system is not necessary to explain the optical conductivity. We believe our data is explained qualitatively by the uncanted magnetic structure with a small offset of the chemical potential from strict stochiometry. There is no definite evidence of bulk Weyl nodes, instead we see signatures of a gapped Dirac dispersion, common in other members of AMnBi$_2$ family or compounds with similar 2D network of Bi atoms.

cond-mat.str-el↗

arXiv2015

Surface Optical and Bulk Acoustic Phonons in the Topological Insulator, Bi2Se2Te

We explore the phonon dynamics of thin films of the topological insulator material Bi2Se2Te using ultrafast pump-probe spectroscopy. The time resolved differential reflectivity of the films exhibit fast and slow oscillations. We have given a careful analysis of variation of phonon frequency as a function of film thickness attributing this to existence of standing acoustic modes. However, no variation in the frequency of the optical phonon modes was found with film thickness. This indicates that the optical phonons intrinsically belong to the surface of the topological insulators. The fact that the acoustic phonons can be tuned by changing the film thickness has tremendous potential for room temperature low power spintronic devices and in topological quantum computation.

cond-mat.mes-hall↗

arXiv2014

Slowing down of vortex motion in thin NbN films near the superconductor-insulator transition

We present a quantitative comparison between the measurements of the complex conductance at low (kHz) and high (GHz) frequency in a thin superconducting film of NbN and the theoretical predictions of the dynamical Beresinksii-Kosterlitz-Thouless theory. While the data in the GHz regime can be well reproduced by extending the standard approach to the realistic case of an inhomogeneous sample, the low-frequency measurements present an anomalously large dissipative response around Tc. This anomaly can only be accounted for by assuming a strong slowing down of the vortex diffusion in the kHz regime, or analogously a strong reduction of the length scale probed by the incoming finite-frequency field. This effect suggests the emergence of an intrinsic length scale for the vortex motion that coincides with the typical size of inhomogeneity probed by STM measurements in disordered NbN films.

cond-mat.supr-con↗