arXiv subjects

Sudip Pal

Publications and source records attributed to Sudip Pal.

17 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↗

arXiv2025

High quality superconducting tantalum resonators with beta phase defects

For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials. Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from $\alpha$-tantalum, avoiding the meta-stable $\beta$-phase that forms when depositing tantalum at room temperature, and has been previously identified to be a source of microwave losses. In this work, we show lumped element resonators containing $\beta$-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors ($Q_\text{i}$) up to $(5.0 \pm 2.5) \times 10^6$ in the single photon regime. They outperform resonators with no sign of the $\beta$-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of $\beta$-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits.

quant-ph↗

arXiv2024

Gapped magnetic ground state in the spin-liquid candidate $\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$ suggested by magnetic spectroscopy

The nature of the magnetic ground state of highly frustrated systems remained puzzling to this day. Here, we have performed multifrequency electron spin resonance (ESR) measurements on a putative quantum spin liquid compound $\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$, which is a rare example of $S = 1/2$ spins on a triangular lattice. At high temperatures, the spin susceptibility exhibits a weak temperature dependence which can be described by the Heisenberg model with an antiferromagnetic exchange interaction of strength $J/k_B \approx 175$ K. At low temperatures, however, the rapid drop of the static spin susceptibility, together with monotonic decrease of the ESR linewidth indicates that strong singlet correlations develop below a pairing energy scale $T^*$ accompanied by a spin gap. On the other hand, a weak Curie-like spin susceptibility and the angular dependence of the linewidth suggest additional contribution from impurity spins. We propose the gradual formation of spin singlets with an inhomogeneous spin gap at low temperatures.

cond-mat.str-el↗

arXiv2024

Size-induced Exchange Bias in Single-phase CoO Nanoparticles

We report exchange bias (EB) in single-phase CoO nanoparticles, where two magnetic phases naturally emerge as the crystallite size decreases from 34.6 to 10.8 nm. The Néel temperature (TN) associated with antiferromagnetic ordering decreases monotonically with the reduction in crystallite size, highlighting the significant influence of size effects. The 34.6 nm nanoparticles exhibit magnetization irreversibility between zero field cooled (ZFC) and field-cooled (FC) states below TN. This irreversibility appears well above TN with further reduction in size, resulting in the absence of true paramagnetic regime which indicates the occurrence of an additional magnetic phase. The frequency-dependent ac-susceptibility in 10.8 nm nanoparticles suggests slow dynamics of disordered surface spins above TN, coinciding with the establishment of long-range order in the core. The thermoremanent magnetization (TRM) and isothermoremanent magnetization (IRM) curves suggest a core-shell structure: the core is antiferromagnetic, and the shell consists of disordered surface spins causing ferromagnetic interaction. Hence, the exchange bias in these CoO nanoparticles results from the exchange coupling between an antiferromagnetic core and a disordered shell that exhibits unconventional surface spin characteristics.

cond-mat.mes-hall↗

arXiv2024

SrCu(OH)$_3$Cl, an ideal isolated equilateral triangle spin $S$ = 1/2 model system

We have investigated the magnetic ground state properties of the quantum spin trimer compound strontium hydroxy copper chloride SrCu(OH)$_3$Cl using bulk magnetization, specific heat measurements, nuclear magnetic resonance (NMR), and electron spin resonance (ESR) spectroscopy. SrCu(OH)$_3$Cl consists of layers with isolated Cu$^{2+}$ triangles and hence provides an opportunity to understand the magnetic ground state of an isolated system of \textit{S} = 1/2 arranged on an equilateral triangle. Although magnetization measurements do not exhibit a phase transition to a long-range ordered state down to \textit{T} = 2 K, they reveal the characteristic behavior of isolated trimers with an exchange of $J = 154$~K. The Curie-Weiss behavior changes around 50--80~K, as is also seen in the NMR spin-lattice relaxation rate. In zero magnetic field, our specific heat data establish a second-order phase transition to an antiferromagnetic ground state below \textit{T}= 1.2 K. We have drawn a magnetic field-temperature ($H$-$T$) phase diagram based on the specific heat measurements. The ESR data show divergence of the linewidth at lower temperatures, which precedes the phase transition to an antiferromagnetic long-range ordered state with unconventional critical exponents. The temperature variation of the $g$-factor further confirms the antiferromagnetic phase transition and reflects the underlying magneto-crystalline anisotropy of the compound.

cond-mat.str-el↗

arXiv2023

Critical-point anomalies in doped CeRhIn5

The heavy-fermion compound CeRhIn$_5$ can be tuned through a quantum critical point, when In is partially replaced by Sn. This way additional charge carriers are introduced and the antiferromagnetic order is gradually suppressed to zero temperature. Here we investigate the temperature-dependent optical properties of CeRh(In$_{1-x}$Sn$_x$)$_5$ single crystals for $x = 4.4\%$, $6.9\%$ and $9.8\%$. With increasing Sn concentration the infrared conductivity reveals a clear enhancement of the $c$-$f$ hybridization strength. At low temperatures we observed a non-Fermi-liquid behavior in the frequency dependence of the scattering rate and effective mass in all three compounds. In addition, below a characteristic temperature $T^* \approx 10$ K, the temperature dependent resistivity $ρ(T)$ follows a $\log T$ behavior, typical for a non-Fermi liquid. The temperature-dependent magnetization also exhibits anomalous behavior below $T^*$. Our investigation reveal that below $T^*$ the system shows a pronounced non-Fermi-liquid behavior and $T^*$ monotonically increases as the quantum critical point is approached.

cond-mat.str-el↗

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

Phase coexistence and associated non-equilibrium dynamics under simultaneously applied magnetic field and pressure

A quantitative estimation of the effect of simultaneously applied external pressure (P) and magnetic field (H) on the phase coexistence has been presented for Pr0.5Ca0.5Mn0.975Al0.025O3 and La0.5Ca0.5MnO3, where the ferromagnetic (FM)-metal and antiferromagnetic (AFM)-insulator phases compete in real space. We found that the nonequilibrium dynamics across the FM-AFM transition is primarily dictated by the effect of P and H on the supercooling, superheating temperatures, and the nucleation and growth rate of the equilibrium phase. These effects across the transition is also responsible for the relative volume fraction of the competing phases at low temperature. Importantly in the entire magnetic field-pressure-temperature range of phase coexistence, the interface between the two competing phases having different spin and structural order plays a very important role in controlling the non-equilibrium dynamics.

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

On certain notions of precompactness, continuity and Lipschitz functions

The underlying theme of this article is a class of sequences in metric structures satisfying a much weaker kind of Cauchy condition, namely quasi-Cauchy sequences (introduced in \cite{bc}) that has been used to define several new concepts in recent articles \cite{PDSPNA2, PDSPNA1}. We first consider a weaker notion of precompactness based on the idea of quasi-Cauchy sequences and establish several results including a new characterization of compactness in metric spaces. Next we consider associated idea of continuity, namely, ward continuous functions \cite{caka}, as this class of functions strictly lies between the classes of continuous and uniformly continuous functions and mainly establish certain coincidence results. Finally a new class of Lipschitz functions called "quasi-Cauchy Lipschitz functions" is introduced following the line of investigations in \cite{Beer1,Beer2,Beer3,g1} and again several coincidence results are proved. The motivation behind such kind of Lipschitz functions is ascertained by the observation that every real valued ward continuous function defined on a metric space can be uniformly approximated by real valued quasi-Cauchy Lipschitz functions.

math.GN↗

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

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↗

arXiv2019

Scaling behavior of Charge order melting in Magnetic field-Pressure-Temperature space of 2.5% Al doped Pr$_{0.5}$Ca$_{0.5}$MnO$_3$

Dc magnetic measurements across the charge ordering (CO) transition temperature (T$_{CO}$) in polycrystalline Pr$_{0.5}$Ca$_{0.5}$Mn$_{0.975}$Al$_{0.025}$O$_3$ have been performed under simultaneous influence of external hydrostatic pressure (P) and magnetic field (H). We show the first experimental evidence that the melting of charge order instability obey an interesting scaling function, $δ$T$_{CO}$/P$^α$ = $f$(H/P$^β$) in H-P-T landscape, where $δ$T$_{CO}$ is the suppression of T$_{CO}$ by P and H. Corresponding values of the exponents, $α$ = 1.63 and $β$ = 0.33 have been extracted from data collapsing phenomena. Possible origin of such a scaling behavior has been discussed.

cond-mat.str-el↗

arXiv2019

Possible glass-like random singlet magnetic state in 1T-TaS2

Two-dimensional layered transition-metal-dichalcogenide compound 1T-TaS2 shows the rare coexistence of charge density wave (CDW) and electron correlation driven Mott transition. In addition, atomic-cluster spins on the triangular lattice of the CDW state of 1T-TaS2 give rise to the possibility of the exotic spin-singlet state in which quantum fluctuations of spins are strong enough to prevent any long range magnetic ordering down to absolute zero ( 0 K). We present here the evidences of a glass-like random singlet magnetic state in 1T-TaS2 at low temperatures through a study of temperature and time dependence of magnetization. Comparing the experimental results with a representative canonical spin-glass system Au(1.8%Mn), we show that this glass-like state is distinctly different from the well established canonical spin-glass state.

cond-mat.str-el↗