arXiv subjects

S. B. Roy

Publications and source records attributed to S. B. Roy.

At least 19 recordsLinked to original sources

arXiv2026

Antiferromagnetic order and magnetic polarons in lightly doped Li$_x$CoO$_2$ (x $\sim$ 0.9)

We investigate the magnetic properties of Li$_x$CoO$_2$ (x$\sim$0.9) using bulk magnetization, specific heat, nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy measurements. The dc magnetization, specific heat and NMR measurements, which probe the macroscopic response, indeed show that this compound partially undergoes an antiferromagnetic transition below $T_N \sim$ 10 K. In addition, we observed a weak ferromagnetic response, which gives rise to the history dependence in magnetization measurements at low fields and is observed at temperatures above room temperature. We propose that there are ferromagnetic clusters at high temperatures due to the formation of magnetic polarons out of doped holes. In EPR measurements performed at the $X$-band frequency, only a fraction of the total spins contribute and show Curie-like paramagnetic behavior as reflected in the temperature dependence of the EPR intensity. The temperature variation of the EPR spectra can be understood in the framework of the diffusion of magnetic polarons.

cond-mat.str-el↗

arXiv2024

Room temperature Mott transistor based on resistive switching in disordered V2O3 films grown on Si

Electric field-induced giant resistive switching triggered by insulator-to-metal transition (IMT) is one of the promising approaches for developing a new class of electronics often referred to as Mottronics. Achieving this resistive switching by minimal external field at room temperature is of paramount research and technological interest. Mott-IMT is often associated with structural modification, which is very important for optoelectronic and actuator applications. Here, we report a giant resistive switching of about 900 % at room temperature in disordered polycrystalline V2O3-Si thin film stabilized at the IMT phase boundary and associated structural transformation under a small electric field. The increase of electron population in the a1g band under the field is responsible for the Mott gap collapse that drives the structural transition. Furthermore, we also fabricated a room temperature Mott-FET with a channel ON/OFF resistive ratio of about 15. This study provides a fundamental mechanism of the Mott-IMT in V2O3 as well as its device applications.

cond-mat.mtrl-sci↗

arXiv2024

Electric field induced Mott-insulator to metal transition and memristive behaviour in epitaxial V$_2$O$_3$ thin film

We report an isothermal electric field-induced first-order phase transition from Mott-insulator to the metallic state in the epitaxial thin film of V$_2$O$_3$ in the temperature regime below its Mott transition temperature $\approx$ 180 K. This isothermal electric field induced transition is accompanied by interesting electro-thermal history effects, which depend on the measurement paths followed in the electric field - temperature phase space. These interesting properties result in tuneable resistive switching and distinct memristive behavior in V$_2$O$_3$. A generalized framework of disorder-influenced first-order phase transition in combination with a resistor network model has been used to explain the observed experimental features. These findings promise possibilities for Mott insulators to be highly energy-efficient switches in novel technologies like neuromorphic computing.

cond-mat.str-el↗

arXiv2023

Geometric Tracking Control of a Multi-rotor UAV for Partially Known Trajectories

This paper presents a trajectory-tracking controller for multi-rotor unmanned aerial vehicles (UAVs) in scenarios where only the desired position and heading are known without the higher-order derivatives. The proposed solution modifies the state-of-the-art geometric controller, effectively addressing challenges related to the non-existence of the desired attitude and ensuring positive total thrust input for all time. We tackle the additional challenge of the non-availability of the higher derivatives of the trajectory by introducing novel nonlinear filter structures. We formalize theoretically the effect of these filter structures on the system error dynamics. Subsequently, through a rigorous theoretical analysis, we demonstrate that the proposed controller leads to uniformly ultimately bounded system error dynamics.

eess.SY↗

arXiv2023

Charge density wave and superconductivity in 6R-TaS2

The layered transition metal dichalcogenide compounds 1T-TaS2 and 4H-TaS2 are well known for their exotic properties, which include charge density wave, superconductivity, Mott transition, etc., and lately quantum spin liquid. Here, we report the magnetic, transport and transmission electron microscopy study of the charge density wave and superconductivity in 6R-TaS2 which is a relatively less studied polymorph of this dichalcogenide TaS2. Our high temperature electron microscopy reveals multiple charge density wave transitions between room temperature and 650K. Magnetization, and the electrical resistivity measurements in the temperature range of 2-400 K reveal that 6R-TaS2 undergoes a charge density wave transition around 305 K and is followed by a transition to a superconducting state around 3.5 K. The low temperature specific heat measurement exhibits anomaly associated with the superconducting transition around 2.4 K. The estimated Ginzburg Landau parameter suggests that this compound lies at the extreme limit of type-II superconductivity.

cond-mat.supr-con↗

arXiv2022

Non-equilibrium magnetic response in concentrated spin-glass AuFe(11%) alloy

We report a detailed study of dc magnetization and ac susceptibility performed on the zero field cooled (ZFC) and field cooled (FC) state of polycrystalline AuFe(11%) alloy. The temperature variation of ZFC and FC dc magnetization at low fields show a distinct peak around Tf = 33 K, which indicates the cooperative freezing of the finite size spin clusters. A weak thermomagnetic irreversibility between ZFC and FC magnetization appears at a temperature Tir, which is slightly below Tf. The ZFC ac susceptibility shows a sharp cusp at Tf, which shifts towards higher temperatures with an increase in the frequency of the ac magnetic field. When ac susceptibility is recorded after cooling the sample from high temperature in the presence of dc bias magnetic field, the susceptibility cusp gets broadened. In this case, no perceptible frequency dependency of the Tf has been observed, but a significant dispersion in the ac susceptibility is present below Tf. This clearly indicates the non-equilibrium nature of the FC state.In addition, the FC state of AuFe(11%) alloy exhibits a pronounced memory effect which further underlines that the FC state is not an equilibrium state. In contrast to the general perception obtained through the meanfield theories of thermodynamic phase transition in spin-glass envisaging the FC state to be an equilibrium state, the present experimental results clearly indicate that the energy landscape of the FC state of AuFe(11%) alloy is a nontrivial one

cond-mat.mtrl-sci↗

arXiv2022

Understanding the magnetic response of the Quantum spin liquid compound 1T-TaS2

1T-tantalum disulphide (1T-TaS2) is a promising material to realize quantum spin liquid state and fractional excitations. Here, we take a closer look on the temperature dependent magnetic susceptibility of 1T-TaS2 at low magnetic fields to understand the formation of local magnetic moment in this system and its response as a function of temperature and field. We found that the susceptibility increases with reduction in the temperature even in the nearly commensurate charge density wave (CDW) phase, which exhibits metallic conductivity and hence is expected to exhibit temperature independent Pauli paramagnetic susceptibility. Therefore, it indicates that local moment starts forming in the nearly commensurate CDW phase itself, which is well above the temperature where the evidence of Mott insulating state is found in resistivity measurement. In the commensurate CDW phase, temperature dependence of susceptibility significantly deviates from the Curie-Weiss law and does not show the magnetic properties expected from S = 1/2 spin on a two dimensional triangular lattice. Below T = 10 K, susceptibility follows a power law dependence on the temperature, which indicates towards the formation of random singlet state in 1T-TaS2.

cond-mat.str-el↗

arXiv2021

Role of electronic excitation on the anomalous magnetism of elemental Copper

Magnetic susceptibility of elemental copper (Cu) shows an anomalous rise at low temperatures superimposed on the expected atypical diamagnetic response. Such temperature dependent susceptibility, which is also known as the Curie tail, can not be explained on the basis of Larmor diamagnetic and Pauli paramagnetic contributions expected in Cu. Using valence band resonant photoemission spectroscopy results and density functional theory calculations, we show the magnetic anomaly appears due to presence of holes in Cu 3d band, which originates from thermally excited electronic configuration. Our study therefore highlights that the Curie tail, which is generally overlooked presuming it either due to paramagnetic impurities or defects, can in fact be intrinsic to a material, and even simple systems like elemental Cu is susceptible to electronic excitations giving rise to anomalous magnetic state.

cond-mat.mtrl-sci↗

arXiv2021

Interesting magnetic response of the nuclear fuel material UO2

Magnetic response of uranium dioxide (UO2) has been investigated through temperature and magnetic field dependent dc magnetization measurements. UO2 is a paramagnet at room temperature. The magnetic susceptibility, however, deviates from Curie-Weiss (CW) like paramagnetic behavior below T = 280 K. Further down the temperature UO2 undergoes phase transition to an antiferromagnetic state below TN = 30.6 K. The zero field cooled (ZFC) and field cooled (FC) magnetizations exhibit some distinct thermomagnetic irreversibility below TN. The temperature dependence of the FC magnetization is more like a ferromagnet, whereas ZFC magnetization exhibits distinct structures not usually observed in the antiferromagnets. In low applied magnetic field this thermomagnetic irreversibility in magnetization exists in a subtle way even in the paramagnetic regime above TN up to a fairly high temperature, but vanishes in high applied magnetic fields. Deviation from CW law and irreversibility between ZFC and FC magnetization indicate that the paramagnetic state above TN is not a trivial one. Magnetic response below TN changes significantly with the increase in the applied magnetic field. Thermomagnetic irreversibility in magnetization initially increases with the increase in the strength of applied magnetic field, but then gets reduced in the high applied fields. A subtle signature of a magnetic field induced phase transition is also observed in the isothermal magnetic field vartaion of magnetization. All these experimetal results highlight the non-trivial nature of the antiferromagnetic state in UO2

cond-mat.str-el↗

arXiv2020

Non-equilibrium magnetic response of canonical spin glass and magnetic glass

Time and history dependent magnetization has been observed in a wide variety of materials, which are collectively termed as the glassy magnetic systems. However, such systems showing similar non-equilibrium magnetic response can be microscopically very different and they can be distinguished by carefully looking into the details of the observed metastable magnetic behavior. Canonical spin glass is the most well studied member of this class and has been extensively investigated both experimentally and theoretically over the last five decades. In canonical spin glasses, the low temperature magnetic state obtained by cooling across the spin glass transition temperature in presence of an applied magnetic field is known as the field cooled (FC) state. This FC state in canonical spin glass is widely believed as an equilibrium state arising out of a thermodynamic second order phase transition. Here, we show that the FC state in canonical spin glass is not really an equilibrium state of the system. We report careful dc magnetization and ac susceptibility measurements on two canonical spin glass systems, AuMn (1.8%) and AgMn (1.1%). The dc magnetization in the FC state shows clear temperature dependence. In addition, the magnetization shows a distinct thermal hysteresis in the temperature regime below the spin glass transition temperature. On the other hand, the temperature dependence of ac susceptibility has clear frequency dispersion below spin glass transition in the FC state prepared by cooling the sample in the presence of a dc-bias field. We further distinguish the metastable response of the FC state of canonical spin glass from the metastable response the FC state in an entirely different class of glassy magnetic system namely magnetic glass, where the non-equilibrium behavior is associated with the kinetic-arrest of a first order magnetic phase transition.

cond-mat.str-el↗

arXiv2020

Localization of electronic states resulting from electronic topological transitions in the Mo$_{1-x}$Re$_x$ alloys: A photoemission study

We present the results of resonant photoemission spectroscopy experiments on the Mo$_{1-x}$Re$_{x}$ alloy compositions spanning over two electronic topological transitions (ETT) at the critical concentrations $x_{C1}$ = 0.05 and $x_{C2}$ = 0.11. The photoelectrons show an additional resonance ($R3$) in the constant initial state (CIS) spectra of the alloys along with two resonances ($R1$ and $R2$) which are similar to those observed in molybdenum. All the resonances show Fano-like line shapes. The asymmetry parameter $q$ of the resonances $R1$ and $R3$ of the alloys is observed to be large and negative. Our analysis suggests that the origin of large negative q is associated with phonon assisted inter band scattering between the Mo-like states and the narrow band that appeared due to the ETT.

cond-mat.supr-con↗

arXiv2020

The field cooled state of canonical spin-glass revisited

Canonical spin-glass (SG) is an enigmatic system in condensed matter physics. In spite of the intense activities of last five decades several questions regarding the nature of the SG phase transition and the SG ground state are yet to be resolved completely. In this backdrop we have revisited the field cooled state of canonical spin-glass. We have experimentally studied magnetic response in two canonical spin-glass systems AuMn(1.8%) and AgMn(1.1%), both in the field cooled (FC) as well as zero field cooled (ZFC) state. We show that the well known magnetic memory effect, which clearly established earlier the metastable nature of the ZFC state in SG, is also present in the FC state. The results of our experimental study indicate that the FC state also is a non-equilibrium state, and hence the energy landscape involved is a non-trivial one. This in turn seriously questions the picture of spin-glass transformation as a second order thermodynamic phase transition.

cond-mat.str-el↗

arXiv2018

Study of Nb$_{0.18}$Re$_{0.82}$ non-centrosymmetric superconductor in the normal and superconducting states

We examine the evidence for multiband superconductivity and non s-wave pairing in the non-centrosymmetric superconductor Nb$_{0.18}$Re$_{0.82}$, using electrical transport, magnetization and specific heat measurements. In the normal state, both the evolution of resistivity with temperature and with magnetic field support a multiband picture. In the superconducting state, the Werthamer, Helfand and Hohenberg (WHH) model cannot adequately describe the temperature dependence of the upper critical field, $H_{c2}(T)$, over the whole temperature range measured. In addition, the observed $H_{c2}(0)$ exceeds the Pauli limit, suggesting non-s-wave pairing. Interestingly, the Kadowaki-Woods ratio and Uemura plot reveal a behavior in Nb$_{0.18}$Re$_{0.82}$ which is similar to that found in unconventional superconductors. The temperature dependence of the lower critical field, $H_{c1}(T)$, follows an anomalous $T^3$ behavior and the derived normalized superfluid density ($ρ_s$) is well explained using a nodeless two-gap description. Phase-fluctuation analysis conducted on the reversible magnetization data, reveals a significant deviation from the mean-field conventional s-wave behavior. This trend is interpreted in terms of a non s-wave spin-triplet component in the pairing symmetry as might be anticipated in a non-centrosymmetric superconductor where anti-symmetric spin-orbit coupling plays a dominant role.

cond-mat.supr-con↗

arXiv2018

Magnetic irreversibility and pinning force density in the Mo$_{100-x}$Re$_x$ alloy superconductors

We have measured the isothermal field dependence of magnetization of the Mo$_{100-x}$Re$_x$ (15 $\leq$ x $\leq$ 48) alloys, and have estimated the critical current and pinning force density from these measurements. We have performed structural characterization of the above alloys using standard techniques, and analyzed the field dependence of critical current and pinning force density using existing theories. Our results indicate that dislocation networks and point defects like voids and interstitial imperfections are the main flux line pinning centres in the Mo$_{100-x}$Re$_x$ alloys in the intermediate fields, i.e., in the small bundle flux line pinning regime. In this regime, the critical current density is also quite robust against increasing magnetic field. In still higher fields, the critical current density is affected by flux creep. In the low field regime, on the other hand, the pinning of the flux lines seems to be influenced by the presence of two superconducting energy gaps in the Mo$_{100-x}$Re$_x$ alloys. This modifies the field dependence of critical current density, and also seems to contribute to the asymmetry in the magnetic irreversibility exhibited by the isothermal field dependence of magnetization

cond-mat.supr-con↗

arXiv2018

Vortex-glass transformation within the surface superconducting state of $β$-phase Mo$_{1-x}$Re$_x$ alloys

We have performed an experimental study on the temperature dependence of electrical resistivity $ρ$($T$) and heat capacity $C$($T$) of the Mo$_{1-x}$Re$_x$ $(x = 0.20, 0.25)$ alloy superconductors in different magnetic fields. In the presence of applied magnetic field, the electrical resistivity of these alloys go to zero at a temperature well above the bulk superconducting transition temperature obtained with the help of heat capacity measurements in the same magnetic field. Our study indicates the presence of surface superconducting state in these alloys, where the flux lines are pinned in the surface sheath of the superconductor. The configuration of the flux-lines (2d pancake-like) in the surface sheath is understood in the realm of the flux-spot model. Experimental evidence in support of the surface mixed-state state or "Kulik vortex-state" and the occurrence of a vortex-liquid to vortex-glass transition is presented.

cond-mat.supr-con↗

arXiv2016

Giant magnetocaloric effect near room temperature in the off-stoichiometric Mn-Co-Ge alloy

We report a giant magnetocaloric effect near room temperature in an off-stoichiometric Mn-Co-Ge alloy, across the magnetostructural transition. The isothermal entropy change accompanying this transition has a peak value of nearly 40 J/kg-K near 297 K and a refrigerant capacity of 270 J/kg with the hot end at 302.5 K and cold end at 293.5 K. We also present an experimental protocol to avoid spurious peaks in the magnetocaloric effect across a sharp first order magnetostructural transition, not confined to Mn-Co-Ge alone, where metastability during the transition could influence the measured magnetization and thus the estimated entropy change. The estimated entropy change in the present off-stoichiometric Mn-Co-Ge alloy is possibly the highest reported value near room temperature in undoped Mn-Co-Ge alloys and underlines the potential of the alloy for technological applications in room temperature magnetic refrigeration.

cond-mat.mtrl-sci↗

arXiv2015

Pinning and elastic properties of vortex matter in highly strained Nb$_{75}$Zr$_{25}$: Analogy with viscous flow of disordered solids

We present the results of magnetization and magneto-transport measurements in the superconducting state of an as-cast Nb$_{75}$Zr$_{25}$ alloy. We also report the careful investigation of the microstructure of our sample at various length scales by using optical, scanning electron and transmission electron microscopies. The information of microstructure is used to understand the flux pinning properties in the superconducting state within the framework of collective pinning. The magneto-transport measurements show a non-Arrhenius behaviour of the temperature and field dependent resistivity in the flux flow region. This non-Arrhenius behaviour is understood in terms of a model, which was originally proposed for viscous flow of disordered solids and is popularly known in the literature as the `shoving' model. The activation energy for flux flow is obtained from magneto-transport measurements and is assumed to be mainly the elastic energy stored in the flux-line lattice. The scaling of pinning force density with respect to reduced field indicates the presence of two pinning mechanisms of different origins. The elastic constants of the flux-line lattice are estimated from magnetization measurements and are used to estimate the length scale of vortex lattice movement, or the volume displaced by the flux-line lattice, during flux flow. It appears that the vortex lattice displacement estimated from elastic energy considerations is of the same order of magnitude as that of the flux-bundle hopping length when a finite resistance appears during flux flow. Our results could provide possible directions for establishing a framework where vortex matter and glass forming liquids or amorphous solids can be treated in a similar manner for understanding the phenomenon of viscous flow in disordered solids or more generally the pinning and depinning properties of elastic manifolds in random media.

cond-mat.supr-con↗

arXiv2014

Phase-coexistence and glass-like behavior in magnetic and dielectric solids with long range order

Phase-coexistence in the manganese-oxide compounds or manganites with colossal magneto-resistance (CMR) has been generally considered to be an inhomogeneous ground state. An alternative explanation of phase-coexistence as the manifestation of a disorder-broadened first order magnetic phase transition being interrupted by the glasslike arrest of kinetics is now gradually gaining ground. This kinetic arrest of a first order phase transitions, between two states with long-range magnetic order, has actually been observed in various other classes of magnetic materials in addition to the CMR manganites. The underlying common features of this kinetic arrest of a first order phase transition are discussed in terms of the phenomenology of glasses. The possible manifestations of such glass-like arrested states across disorder-influenced first order phase transitions in dielectric solids and in multiferroic materials, are also discussed.

cond-mat.mtrl-sci↗