arXiv subjects

Raman Sankar

Publications and source records attributed to Raman Sankar.

At least 19 recordsLinked to original sources

arXiv2026

Observation of Magnetic-Anisotropy Crossover and High-Temperature Skyrmions in the Dirac Magnet Fe3Ge with a Distorted Kagome Lattice

Topological materials that simultaneously host robust high-temperature skyrmions and nontrivial electronic band structures have attracted tremendous interest owing to their distinctive advantages for both fundamental research and prospective technological applications. Here, we report the observation of robust skyrmions in the Dirac kagome magnet Fe3Ge, which exhibits a high Curie temperature of ~ 650 K. At room temperature, Fe3Ge shows a large intrinsic anomalous Hall conductivity of ~ 380 {\Omega}-1cm-1, originating from its nontrivial electronic band topology. Systematic magnetization measurements reveal a spin reorientation transition at ~ 375 K, indicating a crossover from easy-plane to easy-axis magnetic anisotropy. Below the spin reorientation temperature, a large topological Hall effect is observed, arising from microscopic noncoplanar spin structures. Lorentz transmission electron microscopy shows that mesoscopic skyrmions are stabilized in the easy-axis magnetic anisotropy regime and persist over an exceptionally wide temperature window of 375-650 K, far exceeding that of most previously reported skyrmion-hosting materials. These results establish Fe3Ge as a promising platform for exploring diverse topological properties, with strong potential for advancing future high-temperature spintronic applications, ranging from next-generation information storage to logic computing devices.

cond-mat.mtrl-sci↗

arXiv2026

Plaid-Like Spin Splitting and Chirality of Magnon Bands in Antiferromagnetic MnTe$_2$

Altermagnets constitute an emerging class of magnetic materials that combine compensated antiferromagnetic order with spin-split excitations arising from crystalline symmetries. Despite strong theoretical interest, their experimental identification remains challenging. Here, we demonstrate that helicity- and angle-resolved Raman scattering measurements reveal reduced rotational symmetries of magnons and a pronounced imbalance between left- and right-circular polarization channels, indicating momentum-dependent magnon handedness. First-principles DFT+$U$ calculations combined with linear spin-wave theory uncover a characteristic plaid-like spin-splitting structure in momentum space. The resulting magnon spin textures are dictated by the unconventional sublattice symmetries of MnTe$_2$ and closely emulate those of altermagnetic electronic bands. Our work provides evidence of chiral spin-wave excitations unique to this non-coplanar antiferromagnet.

cond-mat.str-el↗

arXiv2026

Pressure-induced reentrant superconductivity in a misfit layered compound $\mathrm{(SnS)_{1.15}(TaS_2)}$

Misfit layered compounds are natural van der Waals heterostructures in which electronically active transition-metal dichalcogenide layers are decoupled by incommensurate blocking layers, enabling bulk realization of quasi-two-dimensional quantum states. Here we investigate the superconducting, transport,and structural properties of the misfit compound $\mathrm{(SnS)_{1.15}(TaS_2)}$ under pressures up to 150 GPa. The low-pressure superconducting phase is gradually suppressed and disappears near 14.7 GPa,accompanied by increasing residual resistance. Remarkably, a distinct superconducting phase reemerges above 80 GPa and persists to the highest pressures achieved. This reentrant superconductivity follows a pressure-induced sign reversal of the Hall coefficient near 60 GPa and a nonmonotonic evolution of the normal-state resistance, indicating an electronic reconstruction. No structural phase transition is detected over the entire pressure range. Our results demonstrate a pressure-driven electronic reconstruction leading to reentrant superconductivity in a misfit layered compound, establishing pressure as an effective route to engineer superconductivity and electronic states in natural van der Waals heterostructures.

cond-mat.supr-con↗

arXiv2026

Tuning magnetic, lattice, and transport properties in CoNb3S6 via Fe doping

We present a comprehensive investigation of the effects of Fe doping on the lattice dynamics, magnetic ordering, and magneto-transport properties of the intercalated van der Waals antiferromagnets Co1-xFexNb3S6 (x = 0.1 and 0.3). Temperature- and polarization-dependent Raman scattering measurements reveal a pronounced blue shift of the 180 cm-1 phonon mode with increasing Fe concentration, indicating enhanced sensitivity of lattice vibrations to Fe-induced structural and mass effects. While the temperature evolution of the phonon modes is dominated by conventional anharmonic phonon softening, subtle anomalies observed near the Néel temperature for x = 0.1 point to weak spin-phonon coupling. Electrical transport and magnetic susceptibility data show clear signatures of the antiferromagnetic phase transitions at TN ~ 20.5-23.7 K for x = 0.1 and TN ~ 32.0 K for x = 0.3. Out-of-plane magnetization measurements reveal hysteretic behavior with two field-induced transitions for x =0.1, which evolve into a single hysteresis loop at x =0.3, signaling a subtle reconstruction of the magnetic ground state. Magneto-transport measurements for x = 0.1 further display a butterfly-shaped hysteretic magnetoresistance and a weak topological Hall effect; however, both features are strongly suppressed at x = 0.3. These results illustrate the critical role of Fe-induced magnetic structure reconstruction in fine-tuning topological and magnetic transport phenomena in intercalated van der Waals antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2025

Field-Angle Dependence of Phonon Thermal Hall Effect in Na2X2TeO6 (X = Co, Zn)

The mechanism behind thermal Hall effects by phonons, which are observed in various materials, is not clarified despite the dominant contribution as heat carriers. Theoretically, mechanisms based on the intrinsic Berry phase and those on extrinsic impurity-induced scatterings have been proposed, which can be distinguished by comparing the field-angle dependence of the thermal Hall effect and that of the magnetic anisotropy. Here, we investigate the field-angle dependence of the thermal Hall effects in the antiferromagnet Na2Co2TeO6 and its non-magnetic isostructural analogue Na2Zn2TeO6 in the ac plane. We find that the field-angle dependence of the thermal Hall conductivity in both materials well follows that of the out-of-plane magnetization, showing a common mechanism by extrinsic impurity-induced scatterings in both the phonon thermal Hall effect and that enhanced by a coupling with the magnetism.

cond-mat.mtrl-sci↗

arXiv2025

Strong and selective magnon-phonon coupling in van der Waals antiferromagnet CoPS$_3$

The Raman scattering response of the biaxial antiferromagnet CoPS$_3$ has been investigated as a function of both magnetic field and temperature. The peaks observed in the low-frequency spectral range (90--200~cm$^{-1}$) have been identified as hybrid magnon--phonon excitations. The energies of the bare magnon and phonon modes, as well as the effective coupling strengths between different excitation pairs, have been determined. The strong and selective magnon--phonon interaction largely accounts for the pronounced splitting of two phonon-like modes observed at 152~cm$^{-1}$ and 158~cm$^{-1}$ in the antiferromagnetic phase of CoPS$_3$. Based on the identification of bare magnon excitations and their magnetic-field dependence, we propose an updated set of parameters for the effective exchange ($J_{\mathrm{eff}} = 9.9$~meV) and biaxial magnetic anisotropy ($D = 4.3$~meV and $E = -0.7$~meV) and advocate for an apparent anisotropic $g$-factor ($g_x = g_y = 2$, $g_z = 4$) in the CoPS$_3$ antiferromagnet.

cond-mat.mes-hall↗

arXiv2025

In-Plane Magnetic Anisotropy and Large topological Hall Effect in Self-Intercalated Ferromagnet Cr1.61Te2

Self-intercalated chromium tellurides Cr1+xTe2 have garnered growing attention due to their high-temperature ferromagnetism, tunable spin structures and air stability, all of which are vital for versatile applications in next-generation memory and information technology. Here, we report strong magnetic anisotropy and a large topological Hall effect (THE) in self-intercalated Cr1.61Te2 single crystals, which are both highly desirable properties for future spintronic applications. Our results demonstrate that Cr1.61Te2 is a soft ferromagnet with strong in-plane magnetic anisotropy. Remarkably, distinct THE behaviors are observed in different temperature regimes, reflecting the intricate spin structures and competing exchange interactions. More interestingly, a large topological Hall resistivity, induced by microscopic non-coplanar spin structures, emerges in the temperature range 70-240 K, reaching a maximum value of 0.93 μΩ cm at 150 K. Moreover, a sign-reversed and weak THE is observed at low temperatures below ~70 K, indicating the emergence of an additional topological spin structure with opposite topological charges. This work not only offers valuable insights into the correlation between magnetocrystalline anisotropy and topological phenomena in Cr1+xTe2 systems, but also provides a robust platform for engineering the evolution of complex spin textures that can be leveraged in diverse spintronic device applications.

cond-mat.mtrl-sci↗

arXiv2025

Direct Visualization of a Disorder Driven Electronic Smectic Phase in Nonsymmorphic Square-Net Semimetal GdSbTe

Electronic liquid crystal (ELC) phases are spontaneous symmetry breaking states believed to arise from strong electron correlation in quantum materials such as cuprates and iron pnictides. Here, we report a direct observation of a smectic phase in a weakly correlated nonsymmorphic square-net semimetal GdSbxTe2-x. Incommensurate smectic charge modulation and intense local unidirectional nanostructure, which coexist with Dirac fermions across Fermi level, are visualized by using spectroscopic imaging - scanning tunneling microscopy. As materials with highly mobile carriers are mostly weakly correlated, the discovery of such an ELC phase are anomalous and raise questions on the origin of their emergence. Specifically, we demonstrate how chemical substitution generates these symmetry breaking phases before the system undergoes a charge density wave (CDW) - orthorhombic structural transition. Our results highlight the importance of impurities in realizing ELC phases and present a new material platform for exploring the interplay among quenched disorder, Dirac fermions and electron correlation.

cond-mat.str-el↗

arXiv2025

Spin waves in Na$_2$Co$_2$TeO$_6$ studied by high-frequency/high-field ESR: Successes and failures of the triple-$\mathbf{q}$ model

The Kitaev candidate material Na$_2$Co$_2$TeO$_6$ is proposed to be proximate to a quantum spin liquid state but a suitable spin model and the nature of its ground states are still under debate. Our high-frequency/high-field electron spin resonance spectroscopy studies of Na$_2$Co$_2$TeO$_6$ single-crystals under in-plane and out-of-plane magnetic fields elucidate the ground state by investigating its low-energy spin wave excitations. Several excitation modes are observed in the low-field phase and in the phases induced by $B\parallel a^*$. In addition, the spectra exhibit a frequency-independent feature at the phase boundary connected to the putative quantum phase transition. For magnetic fields applied along the $c$ axis, the observation of three distinct spin wave modes in the antiferromagnetic (AFM) ground state reveals a previously unresolved splitting of the zero-field excitation gap into $Δ= 211\,$GHz and $Δ_2 = 237\,$GHz. The softening of one of these modes evidences a field-induced phase transition at $B_{\rm c1} = 4.7\,$T, which is corroborated by a clear anomaly in the isothermal magnetization. Spin wave calculations based on the extended Heisenberg-Kitaev model exclude a zigzag ground state of the AFM phase. A triple-q spin configuration correctly predicts two spin wave modes, but fails to reproduce the softening mode. Our analysis shows that the triple-q ground state model of Na$_2$Co$_2$TeO$_6$ is incomplete and suggests the relevance of interlayer interactions.

cond-mat.str-el↗

arXiv2025

Manipulation of anisotropic Zhang-Rice exciton in van der Waals antiferromagnets NiPS3-xSex by anion substitution

Spin-entangled excitons have emerged as intriguing quasi-particle excitations in van der Waals magnets. Among them, the recently observed Zhang-Rice (ZR) exciton in NiPS3 has garnered significant research interest due to its strong correlation with magnetic ordering and its exceptionally long-lived coherence. Herein, we present our in-depth temperature- and polarization-dependent photoluminescence (PL) study of anion-substituted NiPS3-xSex (x=0.008,0.03,0.06 and 0.09 ) to explore the nature and dynamics of the ZR exciton. Our results reveal that, similar to the cation substitution, a small percentage of anion substitution effectively destroys and modulates the ZR exciton, as evidenced by the emergence of a weaker, lower-energy PL peak in addition to the primary ZR peak. The primary and secondary PL peaks exhibit the same anisotropic polarization but differ in their peak energy shift and intensity evolution with Se substitution, suggesting varying charge transfers of p-orbitals. Notably, the ZR exciton undergoes rapid thermal destabilization at much lower temperatures than two-magnon excitations, highlighting that p-orbital inhomogeneity beyond the magnetic ordering structure is a decisive factor in driving its thermal quenching.

cond-mat.mtrl-sci↗

arXiv2025

Strong Long-Wave Infrared Optical Response in a Topological Semiconductor with a Mexican Hat Band Structure

Light sources and photodetectors operating in the far- to mid-infrared (FIR/MIR) band ($8$-$12~\rm μm$, $0.1$-$0.15~\rm eV$) remain relatively poorly developed compared to their counterparts operating in the visible and near-infrared ranges, despite extensive application potential for thermal imaging, standoff sensing, and other technologies. This is attributable in part to the lack of narrow-gap materials ($<0.1~\rm eV$) with high optical gain and absorption. In this work, a narrow-gap semiconductor, $\rm Pb_{0.7}Sn_{0.3}Se$, is demonstrated to exhibit an optical response $>10\times$ larger than that of $\rm Hg_{x}Cd_{1-x}Te$ (MCT), the dominant material for FIR/MIR photodetectors. A previous theoretical investigation indicated that chalcogen $p$ and metal $d$ band inversion in this material creates a Mexican hat band structure (MHBS), which results in a dramatic increase in the joint density of states at the optical transition edge compared to typical semiconductors. This prediction is experimentally validated here using single-crystal specimens of $\rm Pb_{0.7}Sn_{0.3}Se$ measured using temperature-dependent spectroscopic ellipsometry over a wavelength range of $1.7$-$20~\rm μm$ ($0.73$-$0.062~\rm eV$). These measurements demonstrate a large enhancement in extinction coefficient and refractive index characteristic of a MHBS in the vicinity of the absorption edge, in agreement with theoretical predictions. The realization of topological semiconductors with a MHBS is expected to lead to high-efficiency detectors operating in the FIR/MID range.

physics.optics↗

arXiv2024

Interplay between magnetic and lattice excitations and emergent multiple phase transitions in MnPSe3-xSx

The intricate interplay between spin and lattice degrees of freedom in two-dimensional magnetic materials plays a pivotal role in modifying their magnetic characteristics, engendering hybrid quasiparticles, and implementing functional devices. Herein, we present our comprehensive and in-depth investigations on magnetic and lattice excitations of MnPSe3-xSx (x = 0, 0.5, and 1.5) alloys, utilizing temperature- and polarization-dependent Raman scattering. Our experimental results reveal the occurrence of multiple phase transitions, evidenced by notable changes in phonon self-energy and the appearance or splitting of phonon modes. These emergent phases are tied to the development of long and short-range spin-spin correlations, as well as to spin reorientations or magnetic instabilities. Our analysis of two-magnon excitations as a function of temperature and composition showcases their hybridization with phonons whose degree weakens with increasing x. Moreover, the suppression of spin-dependent phonon intensity in chemically most-disordered MnPSe3-xSx (x = 1.5) suggests that chalcogen substitution offers a control knob of tuning spin and phonon dynamics by modulating concurrently superexchange pathways and a degree of trigonal distortions.

cond-mat.mtrl-sci↗

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↗

arXiv2023

A High-Speed Waveguide Integrated InSe Photodetector on SiN Photonics for NIR Applications

On-chip integration of two-dimensional (2D) materials offers great potential for the realization of novel optoelectronic devices in different photonic platforms. In particular, indium selenide (InSe) is a very promising 2D material due to its ultra-high carrier mobility and outstanding photo-responsivity. Here, we report a high-speed photodetector based on a multilayer 90 nm thick InSe integrated on a silicon nitride (SiN) waveguide. The device exhibits a low dark current of 10 nA at 1V bias, a remarkable photoresponsivity of 0.38 AW-1, and high external quantum efficiency of 48.4% measured at 5 V bias. This performance is tested at near-infrared (NIR) 976 nm wavelength under ambient conditions. Furthermore, using numerical and experimental investigations, the estimated absorption coefficient per unit length is 0.11dB/um. To determine the dynamic response of the photodetector, its small and large signal frequency response are also evaluated. A 3-dB radiofrequency (RF) bandwidth of 85 MHz is measured with an open-eye diagram observed at 1 Gbit/s data transmission. Given these outstanding optoelectronic merits, active photonic devices based on integrated multilayer InSe can be realized for a variety of applications including short-reach optical interconnects, LiDAR imaging, and biosensing.

physics.optics↗

arXiv2022

Planar Thermal Hall Effects in Kitaev Spin Liquid Candidate Na2Co2TeO6

We investigate both the longitudinal thermal conductivity ($κ_{xx}$) and the planar thermal Hall conductivity ($κ_{xy}$) in the Kitaev spin liquid candidate of Co-based honeycomb antiferromagnet Na$_2$Co$_2$TeO$_6$ in a magnetic field ($B$) applied along the $a$ and $a^*$ axes. A finite $κ_{xy}$ is resolved for both field directions in the antiferromagnetic (AFM) phase below the Néel temperature of 27 K. The temperature dependence of $κ_{xy}/T$ shows the emergence of topological bosonic excitations. In addition, the field dependence of $κ_{xy}$ shows sign reversals at the critical fields in the AFM phase, suggesting the changes in the Chern number distribution of the topological magnons. Remarkably, a finite $κ_{xy}$ is observed in $B \parallel a^*$ between the first-order transition field in the AFM phase and the saturation field, which is prohibited in a disordered state by the two-fold rotation symmetry around the $a^*$ axis of the honeycomb lattice, showing the presence of a magnetically ordered state that breaks the two-fold rotation symmetry. Our results demonstrate the presence of topological magnons in this compound in the whole field range below the saturation field.

cond-mat.str-el↗

arXiv2021

Observation of a Smoothly Tunable Dirac Point in $Ge(Bi_{x}Sb_{1-x})_{2}Te_{4}$

State-of-the-art topological devices require the use topological surface states to drive electronic transport. In this study, we examine a tunable topological system, $Ge(Bi_{x}Sb_{1-x})_{2}Te_{4}$, for a range of 'x' values from 0 to 1, using a combination of Fourier Transform Scanning Tunneling Spectroscopy (FT-STS) and Angle-Resolved Photoemission Spectroscopy (ARPES). Our results show that the Dirac point shifts linearly with 'x', crossing the Fermi energy near x = 0.7. This novel observation of a smoothly tunable, isolated Dirac point crossing through the topological transport regime and having strong linear dependence with substitution can be critical for future topological spintronics applications.

cond-mat.mtrl-sci↗

arXiv2021

Robust topological state against magnetic impurities observed in superconductor PbTaSe2

Magnetic impurities deposited on topological superconductor candidate PbTaSe2 can introduce a non-splitting zero-energy state inside the superconducting gap, which has been proposed as a field-free platform for topological zero modes. However, it is still unclear how robust the topological state in PbTaSe2 is against magnetic impurities, which is related to the topological nature of the zero-energy state as well as its potential for quantum computation. In this work, we use scanning tunneling microscopy (STM) to study the topological surface state in the normal state of PbTaSe2 under the perturbation of magnetic impurities. We visualize the quasi-particle interference (QPI) arising from the topological surface state. We then deposit Fe impurities on the surface to form atomic Fe adatoms. We find that each Fe adatom sits at a unique interstitial position on the surface and features a local state at high energies, both of which are consistent with our first-principles calculation that further reveals its large magnetic moment. Our systematic Fe deposition and subsequent measurements show that the arc-like QPI pattern at the Fermi energy is robust with up to 3% Fe coverage where the atomic nature of Fe adatoms still holds. Our results provide evidence that the topological surface state at the Fermi energy in PbTaSe2 is robust against dilute magnetic impurities.

cond-mat.supr-con↗

arXiv2021

Pressure-induced excitations in the out-of-plane optical response of the nodal-line semimetal ZrSiS

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS.

cond-mat.mtrl-sci↗