arXiv subjects

S. Aswartham

Publications and source records attributed to S. Aswartham.

At least 19 recordsLinked to original sources

arXiv2026

Nematicity in LaFeAsO single crystals studied by elastoresistance, high-resolution thermal expansion and shear-modulus measurements

Nematicity in LaFeAsO single crystals is studied by means of high-resolution thermal expansion, shear modulus, and elastoresistivity measurements. A softening of the shear modulus $C_{\rm 66}$ towards the structural phase transition at $T_{\rm S}$ is observed. In addition, a similar Curie-Weiss-like divergence of the nematic susceptibilities is found in the temperature dependence of both $\chi^{sh}$ and $\chi^{er}$, which are deduced from the shear modulus (sh) and the elastoresistivity (er) studies, respectively. These observations provide evidence for an electronic origin of nematicity in LaFeAsO. The characteristic energy of the coupling between the lattice and the electronic degrees of freedom is deduced to $\sim$30~K. The comparison to corresponding measurements on BaFe$_2$As$_2$ single crystals reveals a very similar temperature dependence of the shear modulus but yields contrasting results for $\chi^{er}$ : In BaFe$_2$As$_2$, $\chi^{er}$ diverges similarly as the uncoupled nematicity deduced from the shear modulus data as it is expected from the underlying Landau theory. In contrast, the Weiss temperatures of $\chi^{er}$ and $\chi^{sh}$ are significantly different in LaFeAsO. This difference is at odds with the commonly anticipated theories of resistivity anisotropy and electronic nematicity in iron pnictides.

cond-mat.supr-con↗

arXiv2026

Microscopic NMR evidence for successive antiferroelectric and antiferromagnetic order in the van der Waals magnet CuCrP$_2$S$_6$

We present a comprehensive $^{31}$P and $^{65}$Cu nuclear magnetic resonance (NMR) study of the layered van der Waals magnet CuCrP$_2$S$_6$. The compound exhibits a sequence of structural and magnetic phase transitions: a high-temperature paraelectric state, followed by a quasi-antiferroelectric (QAFE) state near 185 K, a long-range antiferroelectric (AFE) phase below 150 K, and finally, antiferromagnetic (AFM) order below $T_\mathrm{N}$ = 30 K. The evolution of the NMR spectra, NMR shift, and spin-lattice ($T_1^{-1}$) and spin-spin ($T_2^{-1}$) relaxation rates provide direct microscopic fingerprints of these transitions. The splitting of both the NMR line and $T_1^{-1}$ below the AFE transition demonstrates the emergence of two inequivalent P sites. From $K - \chi$ analysis, we extract nearly isotropic transferred hyperfine couplings and show that the NMR shift anisotropy originates primarily from the dipolar contribution, in contrast to Mn$_2$P$_2$S$_6$ and Ni$_2$P$_2$S$_6$. We determine the ferromagnetic intralayer exchange $J_{intra}\approx$ -4.9 K from the Curie Weiss temperature, consistent with ferromagnetic layers antiferromagnetically stacked along the $c$ axis, and evaluate the Moriya high temperature relaxation rate including cross correlation effects of the P P dimer. Critical divergence of $T_1^{-1}$ near $T_\mathrm{N}$ yields a critical exponent $\gamma\simeq$ 0.45(4), placing CuCrP$_2$S$_6$ in a three dimensional Heisenberg universality regime.

cond-mat.str-el↗

arXiv2025

Magneto-elasto-resistivity in FeSe

FeSe stands out among iron-based superconductors due to its extended nematic phase without the onset of long-range magnetic order. While strain-dependent electrical resistivity has been extensively explored to probe nematicity, its influence on magneto-transport properties remains less understood. In this work, we present measurements of the magneto-elasto-resistivity in FeSe as a function of temperature and applied magnetic field. Using a minimal multiband Boltzmann model for transport we derive analytical expressions that capture the magnetic behavior of the whole set of experimental data both in the paramagnetic and in the nematic phase. These findings indicate that a multiband framework can robustly describe the magneto-elasto-transport properties in FeSe and arguably in other iron-based superconductors.

cond-mat.supr-con↗

arXiv2025

Point-contact enhanced superconductivity in trigonal PtBi2: quest for the origin of high-Tc

We studied enhanced superconductivity in point contacts (PCs) based on a type-I Weyl semimetal trigonal t-PtBi2 using both normal metal (Ag, Cu, Pt) and ferromagnetic (Fe, Co, Ni) tips by measuring the differential resistance dV/dI(V) curves. In most cases, the value of the superconducting critical temperature Tc ranges between 3 and 5 K, which is several times higher than the maximum bulk Tc. Notably, among the various PCs we examined, a few achieved Tc values as high as 8 K, including those with both normal and ferromagnetic tips. Additionally, the critical magnetic field is also highly enhanced in these PCs and reaches up to several Tesla. The common reason for the Tc increase may be related to pressure/strain caused during the PC's formation. It is worth noting that a greater increase in Tc is observed in PCs formed at the edge of the sample flake, compared to those formed on the plane of the platelet. The results also reveal that the enhancement of Tc in PCs based on t-PtBi2 is compatible with ferromagnetic tip, which may suggest a potentially complex nature of enhanced superconductivity. Our findings besides suggest that t-PtBi2 is a promising candidate for realizing topological superconductivity at more accessible temperatures.

cond-mat.supr-con↗

arXiv2025

Fermi surface evolution in Weyl semimetal t-PtBi$_2$ probed by transverse transport properties

The combination of non-trivial topology and superconductivity opens to novel quantum devices. The discovery of intrinsic materials where such properties appear together represents a frontier in modern condensed matter physics. Trigonal PtBi$_2$ has recently emerged as a possible candidate, being the first example of superconducting type-I Weyl semimetal. However, several aspects of this promising compound still need to be unveiled, concerning its complicated band structure, the actual role of Weyl points in determining its electronic properties and the nature of the superconducting transition. In this work, we experimentally investigated a t-PtBi$_2$ single crystal by means of the Hall and Nernst effects. In particular, we revealed a change of regime in its electronic properties, which is compatible with a temperature and magnetic field evolution of hole-like pockets in the Fermi Surface.

cond-mat.supr-con↗

arXiv2025

Chiral anomaly in the Weyl semimetal TaRhTe$_4$

TaRhTe$_4$ is a type-IIWeyl semimetal, exhibiting fourWeyl points in proximity to the Fermi level. In this article, we report our results of a systematic study of longitudinal magnetoresistance in TaRhTe$_4$. Our findings indicate that magnetoresistance becomes negative only when the magnetic field is applied parallel to the electric field. By rotating E (as well as B), we show that its origin is consistent with the prediction of the chiral anomaly, while the current jetting effect and weak localization could be excluded. The negative magnetoresistance persists up to room temperature, suggesting that TaRhTe4 exhibits distinctive properties within the family of Weyl semimetals.

cond-mat.str-el↗

arXiv2024

Large Nernst effect in Te-based van der Waals materials

Layered van der Waals tellurides reveal topologically non-trivial properties that give rise to unconventional magneto-transport phenomena. Additionally, their semimetallic character with high mobility makes them promising candidates for large magneto-thermoelectric effects. Remarkable studies on the very large and unconventional Nernst effect in WTe$_2$ have been reported, raising questions about whether this property is shared across the entire family of van der Waals tellurides. In this study, systematic measurements of the Nernst effect in telluride van der Waals Weyl semimetals are presented. Large linear Nernst coefficients in WTe$_2$ and MoTe$_2$ are identified, and moderate Nernst coefficients with non-linear behavior in magnetic fields are observed in W$_{0.65}$Mo$_{0.35}$Te$_2$, TaIrTe$_4$, and TaRhTe$_4$. Within this sample set, a correlation between the dominant linear-in-magnetic-field component of the Nernst coefficient and mobility is established, aligning with the established Nernst scaling framework, though with a different scaling factor compared to existing literature. This enhancement might be caused by the shared favorable electronic band structure of this family of materials. Conversely, the non-linear component of the Nernst effect in a magnetic field could not be correlated with mobility. This non-linear term is almost absent in the binary compounds, suggesting a multiband origin and strong compensation between electron-like and hole-like carriers. This comprehensive study highlights the potential of van der Waals tellurides for thermoelectric conversion.

cond-mat.mtrl-sci↗

arXiv2024

Continuous-wave cryogenic optical absorption spectrometer for sub-THz frequencies

We present the design of a continuous-wave, highly sensitive optical spectrometer for millimeter-wave frequencies between 50 and 1000 GHz. The spectrometer uses photomixing of near-infrared light to generate radiation in a wide frequency range, and the absorbed optical power is determined directly through measurements of the sample temperature. This enables relative sensitivities of up to $10^6$ for the sample absorption coefficient below liquid-helium temperatures, suitable for measurements on highly reflective samples. The instrument is also compatible with high magnetic fields. In order to validate its performance, we measure the ferromagnetic resonance in the Mott insulator YTiO$_3$, the electron spin resonance in a standard free-radical reference compound, and the antiferromagnetic resonance in a van der Waals magnetic material.

physics.ins-det↗

arXiv2024

Electron-phonon interaction, magnetic phase transition, charge density waves and resistive switching in VS2 and VSe2 revealed by Yanson point contact spectroscopy

VS2 and VSe2 have attracted particular attention among the transition metals dichalcogenides because of their promising physical properties concerning magnetic ordering, charge density wave (CDW), emergent superconductivity, etc., which are very sensitive to stoichiometry and dimensionality reduction. Yanson point contact (PC) spectroscopic study reveals metallic and nonmetallic states in VS2 PCs, as well as a magnetic phase transition was detected below 25 K. Analysis of PC spectra of VS2 testifies the realization of the thermal regime in PCs. At the same time, rare PC spectra, where the magnetic phase transition was not visible, shows a broad maximum of around 20 mV, likely connected with electron-phonon interaction (EPI). On the other hand, PC spectra of VSe2 demonstrate metallic behavior, which allowed us to detect features associated with EPI and CDW transition. The Kondo effect appeared for both compounds, apparently due to interlayer vanadium ions. Besides, the resistive switching was observed in PCs on VSe2 between a low resistive, mainly metallic-type state, and a high resistive nonmetallic-type state by applying bias voltage (about 0.4V). In contrast, reverse switching occurs by applying a voltage of opposite polarity (about 0.4V). The reason may be the alteration of stoichiometry in the PC core due to the displacement of V ions to interlayer under a high electric field. The observed resistive switching characterize VSe2 as a potential material, e.g., for non-volatile resistive RAM, neuromorphic engineering, and for other nanoelectronic applications. At the same time, VSe2 attracts attention as a rare layered van der Waals compound with magnetic transition.

cond-mat.mes-hall↗

arXiv2023

Interplay of topology and antiferromagnetic order in two-dimensional van der Waals crystals of (NixFe1-x)2P2S6

Mermin-Wagner theorem forbid spontaneous symmetry breaking of spins in one/two-dimensional systems at finite temperature and rules out the stabilization of this ordered state. However, it does not apply to all types of phase transitions in low dimensions such as topologically ordered phase rigorously shown by Berezinskii-Kosterlitz-Thouless (BKT) and experimentally realized in very limited systems such as superfluids, superconducting thin films. Quasi 2D van der Waals magnets provide an ideal platform to investigate the fundamentals of low-dimensional magnetism. We explored the 2D honeycomb antiferromagnetic single crystals of (NixFe1-x)2P2S6 with varying spins ( ) using in depth temperature dependent Raman measurements supported by first-principles calculations of the phonon frequencies. As a function of doping, a tunable transition from paramagnetic to antiferromagnetic ordering is shown via phonons reflected in the strong renormalization of the self-energy parameters of the Raman active phonon modes. An anomalously broad magnetic continuum attributed to two-magnon excitations is observed and its coupling with the phonons is revealed in the observation a Fano line asymmetry. Interestingly, the two-magnon continuum is observed only for the finite doping understood invoking underlying nature of insulator these materials belongs to, i.e. exchange interaction between transition metals via surrounding ligands (sulphur) and the resonance involving phonon modes associated with the (P2S6) cage. Quite surprisingly, we also observed renormalization of the phonon modes much below the long-range magnetic ordered temperature attributed to the topological ordered state, namely the BKT phase, which is also found to change as a function of doping. The extracted critical exponent of the order-parameter evince the signature of topologically active state driven by vortex-antivortex excitations.

cond-mat.mtrl-sci↗

arXiv2023

Peculiarities of electron transport and resistive switching in point contacts on TiSe2, TiSeS and CuxTiSe2

TiSe2 has received much attention among the transition metals chalcogenides because of its thrilling physical properties concerning atypical resistivity behavior, emerging of charge density wave (CDW) state, induced superconductivity etc. Here, we report discovery of new feature of TiSe2, namely, observation of resistive switching in voltage biased point contacts (PCs) based on TiSe2 and its derivatives doped by S and Cu (TiSeS, CuxTiSe2). The switching is taking place between a low resistive mainly metallic-type state and a high resistive semiconducting-type state by applying bias voltage (usually below 0.5V), while reverse switching takes place by applying voltage of opposite polarity (usually below 0.5V). The difference in resistance between these two states can reach up to two orders of magnitude at the room temperature. The origin of the effect can be attributed to the variation of stoichiometry in PC core due to drift/displacement of Se/Ti vacancies under high electric field. Additionally, we demonstrated, that heating takes place in PC core, which can facilitate the electric field induced effect. At the same time, we did not found any evidence for CDW spectral features in our PC spectra for TiSe2. The observed resistive switching allows to propose TiSe2 and their derivatives as the promising materials, e.g., for non-volatile resistive random access memory (ReRAM) engineering.

cond-mat.mes-hall↗

arXiv2023

Magnetic anisotropy and low-energy spin dynamics in the van der Waals compounds Mn$_{2}$P$_{2}$S$_{6}$ and MnNiP$_{2}$S$_{6}$

We report the detailed high-field and high-frequency electron spin resonance (HF-ESR) spectroscopic study of the single-crystalline van der Waals compounds Mn$_{2}$P$_{2}$S$_{6}$ and MnNiP$_{2}$S$_{6}$. Analysis of magnetic excitations shows that in comparison to Mn$_{2}$P$_{2}$S$_{6}$ increasing the Ni content yields a larger magnon gap in the ordered state and a larger g-factor value and its anisotropy in the paramagnetic state. The studied compounds are found to be strongly anisotropic having each the unique ground state and type of magnetic order. Stronger deviation of the g-factor from the free electron value in the samples containing Ni suggests that the anisotropy of the exchange is an important contributor to the stabilization of a certain type of magnetic order with particular anisotropy. At temperatures above the magnetic order, we have analyzed the spin-spin correlations resulting in a development of slowly fluctuating short-range order. They are much stronger pronounced in MnNiP$_{2}$S$_{6}$ compared to Mn$_{2}$P$_{2}$S$_{6}$. The enhanced spin fluctuations in MnNiP$_{2}$S$_{6}$ are attributed to the competition of different types of magnetic order. Finally, the analysis of the temperature dependent critical behavior of the magnon gaps below the ordering temperature in Mn$_{2}$P$_{2}$S$_{6}$ suggests that the character of the spin wave excitations in this compound undergoes a field induced crossover from a 3D-like towards 2D XY regime.

cond-mat.str-el↗

arXiv2022

Evolution of the spin dynamics in the van der Waals system $M_{\text{2}}$P$_{\text{2}}$S$_{\text{6}}$ ($\boldsymbol{M}_{\text{2}}$ = Mn$_{\text{2}}$, MnNi, Ni$_{\text{2}}$) series probed by electron spin resonance spectroscopy

In this work we report a detailed ESR spectroscopic study of the single-crystalline samples of the van der Waals compounds $M_{\text{2}}$P$_{\text{2}}$S$_{\text{6}}$ ($M_{\text{2}}$ = Mn$_{\text{2}}$, MnNi, Ni$_{\text{2}}$), performed at an excitation frequency of 9.56 GHz, in a broad range of temperatures above the magnetic order, and at different orientations of the magnetic field with respect to the sample. Analyzing temperature and angular dependences of the resonance field and of the linewidth of the Mn$_2$P$_2$S$_6$ compound we have observed a significant change of the spin dynamics from the dominance of the 3D-like fluctuations close to the magnetic order to a relative increase of the 2D-like spin fluctuations at higher temperatures. Such a behavior, which is opposite to the development of the low-D signatures in the previously studied Cr$_{\text{2}}$Ge$_{\text{2}}$Te$_{\text{6}}$ compound, can be explained by the difference in the type of magnetic order in Mn$_2$P$_2$S$_6$ and Cr$_{\text{2}}$Ge$_{\text{2}}$Te$_{\text{6}}$. On the other hand, MnNiP$_2$S$_6$ compound exhibits angular dependences of the linewidth typical for the system with 3D-like spin correlations in the whole measurement temperature range, however the 2D-like correlations can be seen in the temperature dependences of the resonance field and the linewidth. Ni$_2$P$_2$S$_6$, in turn, does not show any 2D signatures. This suggests that varying the Ni content in (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ one can control the exchange interaction, possibly also in the third dimension.

cond-mat.str-el↗

arXiv2022

Crystal growth, characterization and electronic band structure of TiSeS

Layered semimetallic van der Waals materials TiSe2 has attracted a lot of attention because of interplay of a charge density wave (CDW) state and superconductivity. Its sister compound TiS2, being isovalent to TiSe2 and having the same crystal structure, shows a semiconducting behavior. The natural rises what happens at the transition point in TiSe2-xSx, which is expected for x close to 1. Here we report the growth and characterization of TiSeS single crystals and the study of the electronic structure using density functional theory (DFT) and angle-resolved photoemission (ARPES). We show that TiSeS single crystals have the same morphology as TiSe2. Transport measurements reveal a metallic state, no evidence of CDW was found. DFT calculations suggest that the electronic band structure in TiSeS is similar to that of TiSe2, but the electron and hole pockets in TiSeS are much smaller. The ARPES results are in good agreement with the calculations.

cond-mat.str-el↗

arXiv2022

Strong uniaxial pressure dependencies evidencing spin-lattice coupling and spin fluctuations in Cr$_2$Ge$_2$Te$_6$

Single crystals of Cr$_2$Ge$_2$Te$_6$ were studied by high-resolution capacitance dilatometry to obtain in-plane ($B\parallel ab$) and out-of-plane ($B\parallel c$) thermal expansion and magnetostriction at temperatures between 2 and 300 K and in magnetic fields up to 15 T. The anomalies in both response functions lead to the 'magnetoelastic' phase diagrams and separate the paramagnetic (PM), ferromagnetic low-temperature/low-field (LTF) and aligned ferromagnetic (FM) phases. The presence of two distinct thermal expansion anomalies at small fields $B\parallel ab$ of different magnetic field dependence clearly supports the scenario of an intermediate region separating PM and LTF phases and is indicative of a tricritical point. Simulations of the magnetostriction using the Stoner-Wohlfarth model for uniaxial anisotropy demonstrate that the observed quadratic-in-field behavior in the LTF phase is in line with a rotation of the spins from the preferred $c$ direction into the $ab$ plane. Both the LTF and the PM phase close to T$_{\rm C}$ exhibit very strong pressure dependencies of the magnetization, ${\partial}\ln{M_{\rm ab}}/{\partial}p_{\rm ab}$, of several hundred %/GPa and also the transition from the LTF to the FM phase strongly depends on $p_{\rm ab}$ ($\sim -$280%/GPa), indicating a strong decrease of the uniaxial anisotropy under applied in-plane pressure. Our data clearly demonstrate the relevance of critical fluctuations and magnetoelastic coupling in Cr$_2$Ge$_2$Te$_6$.

cond-mat.str-el↗

arXiv2022

Electron-phonon interaction and point contact enhanced superconductivity in trigonal PtBi2

PtBi2 is a Weyl semimetal, which demonstrates superconductivity with low critical temperature Tc ~ 0.6 K in the bulk. Here, we report our study of electron-phonon interaction (EPI) in trigonal PtBi2 by the Yanson point contact (PC) spectroscopy and presenting the observation of PC enhanced superconductivity. We show, that the Yansons PC spectra display a broad maximum around 15 meV, indicating, apparently, EPI mechanism of Cooper pairing in PtBi2. Moreover, we discovered a substantial increase of Tc up to ~ 3.5 K in PCs. The observed Tc is sufficiently higher than the bulk value, as well as detected at hydrostatic pressure. We calculated the phonon density of states and Eliashberg EPI function in PtBi2 within the framework of the density functional theory. A comparison of experimental data with theoretical calculations showed acceptable agreement. The theoretical Tc is 3.5 K, which corresponds to the experimental value.

cond-mat.supr-con↗

arXiv2022

Strong effects of uniaxial pressure and short-range correlations in Cr2Ge2Te6

Cr$_2$Ge$_2$Te$_6$ is a quasi-2D semiconducting van der Waals ferromagnet down to the bilayer with great potential for technological applications. Engineering the critical temperature to achieve room temperature applications is one of the critical next steps on this path. Here, we report high-resolution capacitance dilatometry studies on Cr$_2$Ge$_2$Te$_6$ single crystals which directly prove significant magnetoelastic coupling and provide quantitative values of the large uniaxial pressure effects on long-range magnetic order (${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{c}}$ = 24.7 K/GPa and ${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{ab}}$ = $-$15.6 K/GPa) derived from thermodynamic relations. Moderate in-plane strain is thus sufficient to strongly enhance ferromagnetism in Cr$_2$Ge$_2$Te$_6$ up to room temperature. Moreover, unambiguous signs for short-range magnetic order up to 200~K are found.

cond-mat.str-el↗

arXiv2022

Frustration model and spin excitations in the helimagnet FeP

The metallic compound FeP belongs to the class of materials that feature a complex noncollinear spin order driven by magnetic frustration. While its double-helix magnetic structure with a period $\lambda_{\text{s}} \approx 5c$, where $c$ is the lattice constant, was previously well determined, the relevant spin-spin interactions that lead to that ground state remain unknown. By performing extensive inelastic neutron scattering measurements, we obtained the spin-excitation spectra in a large part of the momentum-energy space. The spectra show that the magnons are gapped with a gap energy of $\sim$5 meV. Despite the 3D crystal structure, the magnon modes display strongly anisotropic dispersions, revealing a quasi-one-dimensional character of the magnetic interactions in FeP. The physics of the material, however, is not determined by the dominating exchange, which is ferromagnetic. Instead, the weaker two-dimensional antiferromagnetic interactions between the rigid ferromagnetic spin chains drive the magnetic frustration. Using linear spin-wave theory, we were able to construct an effective Heisenberg Hamiltonian with an anisotropy term capable of reproducing the observed spectra. This enabled us to quantify the exchange interactions in FeP and determine the mechanism of its magnetic frustration.

cond-mat.str-el↗