arXiv subjects

A. Thamizhavel

Publications and source records attributed to A. Thamizhavel.

At least 55 records · Page 3Linked to original sources

arXiv2021

Coplanar cavity for strong coupling between photons and magnons in van der Waals antiferromagnet

We investigate the performance of niobium nitride superconducting coplanar waveguide resonators towards hybrid quantum devices with magnon-photon coupling. We find internal quality factors ~ 20000 at 20 mK base temperature, in zero magnetic field. We find that by reducing film thickness below 100 nm internal quality factor greater than 1000 can be maintained up to parallel magnetic field of ~ 1 T and perpendicular magnetic field of ~ 100 mT. We further demonstrate strong coupling of microwave photons in these resonators, with magnons in chromium trichloride, a van der Waals antiferromagnet, which shows that these cavities serve as a good platform for studying magnon-photon coupling in 2D magnonics based hybrid quantum systems. We demonstrate strong magnon-photon coupling for both optical and acoustic magnon modes of an antiferromagnet.

cond-mat.mes-hall↗

arXiv2020

Superconductivity in amorphous RexZr (x ~ 6) thin films

We report the growth, characterization and superconducting properties of a new amorphous superconductor, RexZr ( x ~ 6 ), in thin film form. Films were grown by pulsed laser deposition with the substrate kept at room temperature. Films with thickness larger than 40 nm showed a superconducting transition temperature (Tc) of 5.9 K. Superconducting properties were measured for films with varying thickness from 120 to 3 nm. The normal state resistance scales linearly with inverse of thickness. The transition temperature, critical field, coherence length, penetration depth and superconducting energy gap changes marginally with decreasing film thickness down to 8 nm. Scanning tunneling spectroscopy and penetration depth measurements provide evidence for a single gap strong coupling s-wave superconductor. Magneto-transport properties indicate a rich magnetic field-temperature phase diagram with the possibility of vortex liquid phases existing over a large fraction of the mixed state.

cond-mat.supr-con↗

arXiv2020

Exceptional Dirac states in a non-centrosymmetric superconductor, BiPd

Quantum materials having Dirac fermions in conjunction with superconductivity is believed to be the candidate materials to realize exotic physics as well as advanced technology. Angle resolved photoemission spectroscopy (ARPES), a direct probe of the electronic structure, has been extensively used to study these materials. However, experiments often exhibit conflicting results on dimensionality and momentum of the Dirac Fermions (e.g. Dirac states in BiPd, a novel non-centrosymmetric superconductor), which is crucial for the determination of the symmetry, time-reversal invariant momenta and other emerging properties. Employing high-resolution ARPES at varied conditions, we demonstrated a methodology to identify the location of the Dirac node accurately and discover that the deviation from two-dimensionality of the Dirac states in BiPd proposed earlier is not a material property. These results helped to reveal the topology of the anisotropy of the Dirac states accurately. We have constructed a model Hamiltonian considering higher-order spin-orbit terms and demonstrate that this model provides an excellent description of the observed anisotropy. Intriguing features of the Dirac states in a non-centrosymmetric superconductor revealed in this study expected to have significant implication in the properties of topological superconductors.

cond-mat.supr-con↗

arXiv2020

Evidence of non-trivial Berry phase and Kondo physics in SmBi

Realization of semimetals with non-trivial topologies such as Dirac and Weyl semimetals, have provided a boost in the study of these quantum materials. Presence of electron correlation makes the system even more exotic due to enhanced scattering of charge carriers, Kondo screening etc. Here, we studied the electronic properties of single crystalline, SmBi employing varied state of the art bulk measurements. Magnetization data reveals two magnetic transitions; an antiferromagnetic order with a Neel temperature of ~ 9 K and a second magnetic transition at a lower temperature (= 7 K). The electrical resistivity data shows an upturn typical of a Kondo system and the estimated Kondo temperature is found to be close to the Neel temperature. High quality of the crystal enabled us to discover signature of quantum oscillation in the magnetization data even at low magnetic field. Using a Landau level fan diagram analysis, a non-trivial Berry phase is identified for a Fermi pocket revealing the topological character in this material. These results demonstrate an unique example of the Fermiology in the antiferromagnetic state and opens up a new paradigm to explore the Dirac fermion physics in correlated topological metal via interplay of Kondo interaction, topological order and magnetism.

cond-mat.str-el↗

arXiv2020

Structural anomaly in superconductivity of CaFe2As2 class of materials

Quantum transitions in Fe-based systems are believed to involve spin, charge and nematic fluctuations. Complex structural phase diagram in these materials often emphasizes importance of covalency in their exotic properties, which is directly linked to the local structural network and barely understood. In order to address this outstanding issue, we investigate the evolution of structural parameters and their implication in unconventional superconductivity of 122 class of materials employing extended x-ray absorption fine structure studies. The spectral functions near the Fe K- and As K-absorption edge of CaFe2As2 and its superconducting composition, CaFe1.9Co0.1As2 (Tc = 12 K) exhibit evidence of enhancement of Fe contribution with Co-substitution near the Fermi level. As-Fe and Fe-Fe bondlengths derived from the experimental data exhibit interesting changes with temperature across the magneto-structural transition. Curiously, the evolution in Co-doped composition is similar to its parent compound despite absence of magneto-structural transition. In addition, we discover anomalous change of Ca-X (X = Fe, As) bondlengths with temperature in the vicinity of magneto-structural transition and disorder appears to be less important presumably due to screening by the charge reservoir layer. These results reveal evidence of doping induced evolution to the proximity to critical behavior presumably leading to superconductivity in the system.

cond-mat.supr-con↗

arXiv2020

Preparation, characterization and X-ray photoemission spectroscopy study of a correlated semimetal, SmBi

We report high quality single crystalline growth of SmBi using flux method. The compound crystallizes in the simple rock salt structure with space group Fm3m. The cubic structure of the single crystal was confirmed by Laue diffraction pattern. The magnetic susceptibility measurements reveal sharp antiferromagnetic order with Neel temperature of 9 K. The core level photoemission spectroscopy study of Sm 3d has been performed using monochromatic Al Kalpha source. We observe multiple features in the experimental spectra due to fonal state effect - a signature of hybridization between Sm 4f - Bi 6p states. Intense satellite features are also observed presumably due to mixed valency arising from Kondo coupling. No signature of surface-bulk difference is observed in the Sm 3d core level spectra.

cond-mat.str-el↗

arXiv2020

Unusual role of ligand states in the electronic properties of a parent Fe-based superconductor, CaFe2As2

We investigate the role of ligand states in the electronic properties of CaFe2As2 using high-resolution hard x-ray photoemission spectroscopy (HAXPES) at different sample temperatures. Experimental results indicate that the binding energy of Ca is close to that for 2+ charge state of Ca atoms and the other constituent elements, Fe and As possess electronic configuration close to that in elemental systems. No difference is observed in the As 3p core level spectra with the change of emission angle and/or the change in sample temperature. This is surprising as the Ca atoms at the cleaved sample surface reorganizes itself to form linear structures which is expected to influence Ca-As hybridization leading to significant difference in surface and bulk electronic structures. Moreover, CaFe2As2 undergoes structural and magnetic phase transition at 170 K, and strong Fe-As hybridization provides pathways for electron dynamics. Clearly, further studies are required to resolve these puzzling observations.

cond-mat.supr-con↗

arXiv2020

Anomalous spectral evolution with bulk sensitivity in BiPd

We investigate the electronic structure of a noncentrosymmetric superconductor, BiPd using photoemission spectroscopy with multiple photon energies ranging from ultraviolet to hard x-ray. Experimental data exhibit interesting difference in the surface and bulk electronic structures of this system. While the surface Bi core level peaks appear at lower binding energies, the surface valence band features are found at the higher binding energy side of the bulk valence band; valence band is primarily constituted by the Pd 4d states. These changes in the electronic structure cannot be explained by the change in ionicity of the constituent elements via charge transfer. Analysis of the experimental data indicates that the Bi-Pd hybridization physics plays the key role in deriving the anomalous spectral evolution and the electronic properties of this system.

cond-mat.mtrl-sci↗

arXiv2020

Preparation, Characterization and electronic structure of Ti-doped Bi$_2$Se$_3$

We report the preparation of high-quality single crystal of Bi$_2$Se$_3$, a well-known topological insulator and its Ti-doped compositions using Bridgeman technique. Prepared single crystals were characterized by x-ray diffraction (XRD) to check the crystalline structure and energy dispersive analysis of x-rays for composition analysis. The XRD data of Ti-doped compounds show a small shift with respect to normal Bi$_2$Se$_3$ indicating changes in the lattice parameters while the structure type remained unchanged; this also establishes that Ti goes to the intended substitution sites. All the above analysis establishes successful preparation of these crystals with high quality using Bridgman technique. We carried out x-ray photo-emission spectroscopy to study the composition via investigating the core level spectra. Bi$_2$Se$_3$ spectra exhibit sharp and distinct features for the core levels and absence of impurity features. The core level spectra of the Ti-doped sample exhibit distinct signal due to Ti core levels. The analysis of the spectral features reveal signature of plasmon excitation and final state satellites; a signature of finite electron correlation effect in the electronic structure.

cond-mat.mtrl-sci↗

arXiv2020

Selective spin and nematic excitations using polarized pump pulses

Quantum materials display exotic behaviours related to the interplay between temperature-driven phase transitions. Here, we study electron dynamics in one such material, CaFe2As2, a parent Fe-based superconductor, employing time and angle-resolved photoemission spectroscopy. CaFe2As2 exhibits concomitant transition to spin density wave state and nematic order below 170 K. We discover that magnetic excitations can be induced selectively, by using polarized pump pulses, significantly before the destruction of nematic order. More specifically, we observe that s-polarized light enhances electron temperature by several hundreds of degrees at a time scale of about 200 fs, while the temperature of the electrons participating in magnetic order can be enhanced by similar amount at a much faster time scale (50 fs) using p-polarized pump pulse. These results provide a pathway to achieve selective electron heating, which is not possible with other methods, as well as to disentangle phase transitions in quantum materials.

cond-mat.supr-con↗

arXiv2020

Evolution of charge density wave order and superconductivity under pressure in LaPt$_2$Si$_2$

We report measurements of the electrical resistivity and ac magnetic susceptibility of single crystalline LaPt$_2$Si$_2$ under pressure, in order to investigate the interplay of superconductivity and CDW order. LaPt$_2$Si$_2$ exhibits a first order phase transition from a tetragonal to orthorhombic structure, accompanied by the onset of CDW order below $T_{\rm{CDW}}$ = 76 K, while superconductivity occurs at a lower temperature of $T_{\rm{c}}$ = 1.87 K. We find that the application of pressure initially suppresses the CDW transition, but enhances $T_{\rm{c}}$. At pressures above 2.4 GPa, CDW order vanishes, while both $T_{\rm{c}}$ and the resistivity $A$-coefficient reach a maximum value around this pressure. Our results suggest that the occurrence of a superconducting dome can be accounted for within the framework of BCS theory, where there is a maximum in the density of states upon the closure of the CDW gap.

cond-mat.supr-con↗

arXiv2020

On-demand local modification of high-$T_\text{c}$ superconductivity in few unit-cell thick Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

High-temperature superconductors (HTS) are important for potential applications and for understanding the origin of strong correlations. Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO), a van der Waals material, offers a platform to probe the physics down to a unit-cell. Guiding the flow of electrons by patterning 2DEGS and oxide heterostructures has brought new functionality and access to new science. Similarly, modifying superconductivity in HTS locally, on a small length scale, will be of immense interest for superconducting electronics. Here we report transport studies on few unit-cell thick BSCCO and modify its superconductivity locally by depositing metal on the surface. Deposition of chromium (Cr) on the surface over a selected area of BSCCO results in insulating behavior of the patterned region. Cr locally depletes oxygen in CuO$_2$ planes and disrupts the superconductivity in the layers below. Our technique of modifying superconductivity is suitable for making sub-micron superconducting wires and more complex superconducting electronic devices.

cond-mat.supr-con↗

arXiv2020

Evidence of standing spin-waves in a van der Waals magnetic material

Spin-waves have been studied for data storage, communication and logic circuits in the field of spintronics based on their potential to substitute electrons. Recent discovery of magnetism in two-dimensional (2D) systems such as monolayer CrI$_3$ and Cr$_2$Ge$_2$Te$_6$ has led to a renewed interest in such applications of magnetism in the 2D limit. Here we present direct evidence of standing spin-waves along with the uniform precessional resonance modes in van der Waals magnetic material, CrCl$_3$. Our experiment is the first direct observation of standing spin-wave modes, set up across a thickness of 20 $μ$m, in a van der Waals material. We detect standing spin-waves in the vicinity of both, optical and acoustic, branches of the antiferromagnetic resonance. We also observe magnon-magnon coupling, softening of resonance modes with temperature and extract the evolution of interlayer exchange field as a function of temperature.

cond-mat.mtrl-sci↗

arXiv2019

Anisotropic magnetic properties of trigonal ErAl$_2$Ge$_2$ single crystal

We report the anisotropic magnetic properties of the ternary compound ErAl$_2$Ge$_2$. Single crystals of this compound were grown by high temperature solution growth technique,using Al:Ge eutectic composition as flux. From the powder x-ray diffraction we confirmed that ErAl$_2$Ge$_2$ crystallizes in the trigonal CaAl$_2$Si$_2$-type crystal structure. The anisotropic magnetic properties of a single crystal were investigated by measuring the magnetic susceptibility, magnetization, heat capacity and electrical resistivity. A bulk magnetic ordering occurs around 4 K inferred from the magnetic susceptibility and the heat capacity. The magnetization measured along the $ab$-plane increases more rapidly than along the $c$-axis suggesting the basal $ab$-plane as the easy plane of magnetization. The magnetic susceptibility, magnetization and the $4f$-derived part of the heat capacity in the paramagnetic regime analysed based on the point charge model of the crystalline electric field (CEF) indicate a relatively low CEF energy level splitting.

cond-mat.str-el↗

arXiv2019

Weak antilocalization in a noncentrosymmetric CaAgBi single crystal

We report on the single crystal growth and transport properties of a topological semimetal CaAgBi which crystallises in the hexagonal $ABC-$type structure with the non-centrosymmetric space group $\mathit{P6_3mc}$ (No. 186). The transverse magnetoresistance measurements with current in the basal plane of the hexagonal crystal structure reveal a value of about 30 % for I // [10-10] direction and about 50 % for I // [1-210] direction at 10 K in an applied magnetic field of 14 T. The magnetoresistance shows a cusp-like behavior in the low magnetic-field region, suggesting the presence of weak antilocalization effect for temperatures less than 100 K. The Hall measurements reveal that predominant charge carriers are $p$ type exhibiting a linear behavior for fields up to 14 T and can be explained based on the single band model. The magnetoconductance of CaAgBi is analysed based on the modified Hikami-Larkin-Nagaoka (HLN) model. Our first-principles calculations within a density-functional theory framework reveal that CaAgBi supports a topological Dirac semimetal state with Dirac points located on the rotational axis slightly above the Fermi level and are protected by $C_{6v}$ point-group symmetry. The Fermi surface consists of both the electron and hole pockets. However, the size of hole pockets is much larger than electron pockets suggesting the dominant $p$ type carriers in accord with our experimental results.

cond-mat.mtrl-sci↗

arXiv2019

Complex hybridization physics and evidence of structural anomaly to be a bulk property in an exotic Fe-based compound, CaFe2As2

Surface of quantum materials often exhibits significantly different behavior than the bulk due to changed topologies and symmetry protections. The outstanding problem is to find out if the exoticity of a material is linked to the changed topology at the surface or it is a bulk property. Hard x-ray photoemission spectroscopy (HAXPES) is a significantly bulk sensitive technique (escape depth of valence electrons is about 40 Å for 6 keV photon energy) and the probing depth can be tuned by changing the electron emission angle. Therefore, HAXPES is often used to reveal the surface-bulk differences in a material. Here, we show that the delineation of surface-bulk differences in the valence band spectral functions using this method is highly non-trivial due to the complexity arising from linear dichroic effect in addition to the change in surface sensitivity. We show that core level spectra can be used to reveal the surface-bulk differences in the electronic structure. The Ca 2p spectra exhibit evidence of significant hybridization with the conduction electrons revealing their importance in the electronic properties of the system as also found for the charge reservoir layers in cuprate superconductors. The Fe 2p core level spectra as a function of bulk sensitivity and temperature reveals an unusual scenario; while the surface electronic structure corroborates well with the observed phase transitions of the system, the bulk spectra exhibit signature of additional structural phases providing a rare evidence of structural anomaly to be a bulk property.

cond-mat.str-el↗

arXiv2019

Anomalies in the core level spectroscopy of a noncentrosymmetric solid, BiPd

Understanding exotic solids is a difficult task as interactions are often hidden by the symmetry of the system. Here, we study the electronic properties of a noncentrosymmetric solid, BiPd, which is a rare material exhibiting both superconductivity and topological phase of matter. Employing high resolution photoemission spectroscopy with photon energies ranging from hard x-ray to extreme ultraviolet regime, we show that hard x-ray spectroscopy alone is not enough to reveal surface-bulk differences in the electronic structure. We derived the escape depths close to the extreme surface sensitivity and find that the photon energies used for high resolution measurements such as ARPES fall in the surface sensitive regime. In addition, we discover deviation of the branching ratio of Bi core level features derived from conventional quantum theories of the core hole final states. Such paradigm shift in core level spectroscopy can be attributed to the absence of center of symmetry and spin-orbit interactions.

cond-mat.mtrl-sci↗

arXiv2019

Coexistence of Eu-antiferromagnetism and pressure-induced superconductivity in EuFe2As2 single crystal

By performing high-pressure single-crystal neutron diffraction measurements, the evolution of structure and magnetic ordering in EuFe2As2 under hydrostatic pressure were investigated. Both the tetragonal-toorthorhombic structural transition and the Fe spin-density-wave (SDW) transition are gradually suppressed and become decoupled with increasing pressure. The antiferromagnetic order of the Eu sublattice is, however, robust against the applied pressure up to 24.7 kbar, without showing any change of the ordering temperature. Under the pressure of 24.7 kbar, the lattice parameters of EuFe2As2 display clear anomalies at 27(3) K, well consistent with the superconducting transition observed in previous high-pressure resistivity measurements. Such an anomalous thermal expansion around Tc strongly suggests the appearance of bulk superconductivity and strong electron-lattice coupling in EuFe2As2 induced by the hydrostatic pressure. The coexistence of long-range ordered Eu-antiferromagnetism and pressure-induced superconductivity is quite rare in the EuFe2As2-based iron pnictides.

cond-mat.supr-con↗