arXiv subjects

Arumugam Thamizhavel

Publications and source records attributed to Arumugam Thamizhavel.

At least 37 records · Page 2Linked to original sources

arXiv2022

Coupling between colossal charge density wave ordering and magnetism in Ho2Ir3Si5

Ho2Ir3Si5 belongs to the family of three-dimensional (3D) R2Ir3Si5 (R = Lu, Er and Ho) compounds that exhibit a colossal first-order charge density wave (CDW) transition where there is a strong orthorhombic-to-triclinic distortion of the lattice accompanied by superlattice reflections. The analysis by single-crystal X-ray diffraction (SXRD) has revealed that the Ir-Ir zigzag chains along c are responsible for the CDW in all three compounds. The replacement of the rare earth element from non-magnetic Lu to magnetic Er or Ho lowers TCDW, where TCDWLu = 200 K, TCDWEr = 150 K and TCDWHo = 90 K. Out of the three compounds, Ho2Ir3Si5 is the only system where second-order superlattice reflections could be observed, indicative of an anharmonic shape of the modulation wave. The CDW transition is observed as anomalies in the temperature dependencies of the specific heat, electrical conductivity and magnetic susceptibility, which includes a large hysteresis of 90 to 130 K for all measured properties, thus corroborating the SXRD measurements. Similar to previously reported Er2Ir3Si5, there appears to be a coupling between CDW and magnetism such that the Ho3+ magnetic moments are influenced by the CDW transition, even in the paramagnetic state. Moreover, earlier investigations on polycrystalline material revealed antiferromagnetic (AFM) ordering at TN = 5.1 K, whereas AFM order is suppressed and only the CDW is present in our highly ordered single-crystal. First-principles calculations predict Ho2Ir3Si5 to be a metal with coexisting electron and hole pockets at the Fermi level. The Ho and Ir atoms have spherically symmetric metallic-type charge density distributions that are prone to CDW distortion. Phonon calculations affirm that the Ir atoms are primarily responsible for the CDW distortion, which is in agreement with the experiment.

cond-mat.str-el↗

arXiv2022

Orthorhombic charge density wave on the tetragonal lattice of EuAl4

EuAl4 possesses the BaAl4 crystal structure type with tetragonal symmetry I4/mmm. It undergoes a charge-density-wave (CDW) transition at TCDW = 145 K and it features four consecutive antiferromagnetic phase transitions below 16 K. Here, we use single-crystal x-ray diffraction to determine incommensurately modulated crystal structure of EuAl4 in its CDW state. The CDW is shown to be incommensurate with modulation wave vector q = (0, 0, 0.1781(3)) at 70 K. The symmetry of the incommensurately modulated crystal structure is orthorhombic with superspace group Fmmm(00σ)s00, where Fmmm is a subgroup of I4/mmm of index 2. Both the lattice and the atomic coordinates of the basic structure remain tetragonal. Symmetry breaking is entirely due to the modulation wave, where atoms Eu and Al1 have displacements exclusively along a, while the fourfold rotation would require equal displacement amplitudes along a and b. The calculated band structure of the basic structure and interatomic distances in the modulated crystal structure both indicate the aluminum atoms as location of the CDW. The temperature dependence of the specific heat reveals an anomaly at TCDW = 145 K of a magnitude similar to canonical CDW systems. The present discovery of orthorhombic symmetry for the CDW state of EuAl4 leads to the suggestion of monoclinic instead of orthorhombic symmetry for the third AFM state.

cond-mat.str-el↗

arXiv2022

Terahertz Optical Properties and Birefringence in Single Crystal Vanadium doped [100] \b{eta}-Ga2O3

We report the Terahertz optical properties of the Vanadium doped [100] \b{eta}-Ga2O3 using Terahertz Time-Domain Spectroscopy (THz-TDS). The V-doped \b{eta}-Ga2O3 crystal shows strong birefringence in the 0.2-2.4 THz range. Further, phase retardation by the V-doped \b{eta}-Ga2O3 has been measured over the whole THz range by Terahertz Time-Domain Polarimetry (THz-TDP). It is observed that the V-doped \b{eta}-Ga2O3 crystal behaves both as a quarter waveplate (QWP) at 0.38, 1.08, 1.71, 2.28 THz, and a half waveplate (HWP) at 0.74 and 1.94 THz, respectively.

cond-mat.mtrl-sci↗

arXiv2021

Antiferromagnetism and large magnetoresistance in GdBi single crystal

Single crystal of the binary equi-atomic compound GdBi crystallizing in the rock salt type cubic crystal structure with the space group $Fm\bar{3}m$ has been grown by flux method. The electrical and magnetic measurements have been performed on well oriented single crystals. The antiferromagnetic ordering of the Gd moments is confirmed at $T_{\rm N} = 27.5$~K. The magnetization measurement performed at $2$~K along the principal crystallographic direction [100] did not show any metamagnetic transition and no sign of saturation up to $7$~T. Zero field electrical resistivity reveals a sharp drop at $27.5$~K suggesting a reduction in the spin disorder scattering due to the antiferromagnetic alignment of the Gd moments. The residual resistivity at $2$~K is 390~n$Ω$cm suggesting a good quality of the grown crystal. The magneto resistance attains a value of $1.0~\times~10^{4}\%$ with no sign of saturation, in a field of $14$~T, at $T = 2$~K. Shubnikov de Hass (SdH) oscillations have been observed in the high field range of the magnetoresistance with five different frequencies corresponding to the extremal areas of the Fermi surface. Analysis of the Hall data revealed a near compensation of the charge carriers accounting for the extremely large magnetoresistance.

cond-mat.str-el↗

arXiv2021

Growth and characterization of high-quality single-crystalline SnTe retaining cubic symmetry down to the lowest temperature studied

SnTe, an archetypical topological crystalline insulator, often shows a transition from a highly symmetric cubic phase to a rhombohedral structure at low temperatures. In order to achieve the highly symmetric cubic phase at low temperatures suitable for quantum behaviour, we have employed the modified Bridgman method to grow a high-quality single-crystalline sample of SnTe. Analysis of the crystal structure using Laue diffraction and rocking curve measurements show a very high degree of single crystallinity of the sample. Resistivity and the specific heat data do not show the signature of structural transition down to the lowest temperature studied. The magnetic susceptibility shows diamagnetic behaviour. All these properties manifest the behaviour of a typical bulk semiconductor with conducting surface states as expected in a topological material. Detailed powder x-ray diffraction measurements show cubic structure in the whole temperature range studied.

cond-mat.mtrl-sci↗

arXiv2021

Linear unsaturated magnetoresistance in YSi single crystal

Linear magnetoresistance is a phenomenon that has been observed in a few topological compounds that originate from classical and quantum phenomena. Here, we performed electrical transport measurements, in zero and applied magnetic fields, on the YSi single crystal along all three principal crystallographic directions of the orthorhombic crystal structure. For $I~\parallel~[001]$ and $H~\parallel~[100]$ direction above $\approx 10$~T, mobility fluctuation driven linear magnetoresistance is observed without any sign of saturation up to $14$~T magnetic field. Anisotropy in the Fermi surface is immanent from the angular dependence of the magnetoresistance. Kohler rule violation is observed in this system and Hall data signifies multiple charge carriers in YSi.

cond-mat.str-el↗

arXiv2021

Synthesis and magnetic properties of spin-$\frac{3}{2}$ $γ$-phase of SrCo$_{2}$(PO$_{4}$)$_{2}$ antiferromagnet

We report on the synthesis of a new $γ$-phase of the spin $S$~=~$\frac{3}{2}$ compound SrCo$_2$(PO$_4$)$_2$ together with a detailed structural, magnetic and thermodynamic properties. The $γ$-phase of SrCo$_2$(PO$_4$)$_2$ crystallizes in a triclinic crystal structure with the space group $P\bar{1}$. Susceptibility and specific heat measurements reveal that SrCo$_2$(PO$_4$)$_2$ orders antiferromagnetically below $T_{\rm N}\simeq 8.5$\,K and the nature of ordering is three dimensional (3D). The magnetic isotherm at temperatures below $T_{\rm N}$ shows a field-induced spin-flop transition, related to the magnetocrystalline anisotropy, at an applied field of $\sim$~4.5~Tesla. Remarkably, heat capacity shows magnetic-field-induced transitions at $T_{\rm N1}$ = 3.6 K and $T_{\rm N2}$ = 7.4 K. The magnetic long range ordering (LRO) is also confirmed in both the Knight shift and spin-lattice relaxation rate ($1/T_{1}$) of the $^{31}$P-NMR measurements. However, below the LRO we have not detected any NMR signal due to faster relaxation. We have detected two structurally different phosphorous sites in $γ$-phase of SrCo$_{2}$(PO$_{4}$)$_{2}$ and they shift differently with temperature.

cond-mat.str-el↗

arXiv2020

Destruction of superconductivity through phase fluctuations in ultrathin a-MoGe films

We investigate the evolution of superconductivity with decreasing film thickness in ultrathin amorphous MoGe (a-MoGe) films using a combination of sub-Kelvin scanning tunneling spectroscopy, magnetic penetration depth measurements and magneto-transport measurements. We observe that superconductivity is strongly affected by quantum and classical phase fluctuations for thickness below 5 nm. The superfluid density is strongly suppressed by quantum phase fluctuations at low temperatures and evolves towards a linear-T dependence at higher temperatures. This is associated with a rapid decrease in the superconducting transition temperature, Tc, and the emergence of a pronounced pseudogap above Tc. These observations suggest that at strong disorder the destruction of superconductivity follows a Bosonic route where the global superconducting state is destroyed by phase fluctuations even though the pairing amplitude remains finite.

cond-mat.supr-con↗

arXiv2020

Anisotropic Physical Properties of the Kondo Semimetal CeCu$_{1.11}$As$_2$

The recently proposed novel materials class called Weyl-Kondo semimetal (WKSM) is a time reversal invariant but inversion symmetry broken Kondo semimetal in which Weyl nodes are pushed to the Fermi level by the Kondo interaction. Here we explore whether CeCu$_{1+x}$As$_2$ may be a new WKSM candidate. We report on its single-crystal growth, structure determination and physical properties investigation. Previously published studies on polycrystalline samples suggest that it is indeed a Kondo semimetal, which is confirmed by our investigations on single crystals. X-ray diffraction reveals that CeCu$_{1+x}$As$_2$ crystallizes in a tetragonal centrosymmetric structure, although the inversion symmetry could still be broken locally due to partially occupied Cu sites. Chemical analysis results in an average occupation $x$ = 0.11(1). The electrical resistivity increases logarithmically with decreasing temperature, and saturates below 10 K. A Kondo temperature $T_{\mathrm{K}}$ $\approx$ 4 K is extracted from entropy, estimated from the specific heat measurements. From Hall effect experiments, a charge carrier density of $8.8 \times 10^{20}$ cm$^{-3}$ is extracted, a value characteristic of a semimetal. The magnetization shows pronounced anisotropy, with no evidence of magnetic ordering down to 0.4 K. We thus classify CeCu$_{1.11}$As$_2$ as a tetragonal Kondo semimetal with anisotropic magnetic properties, with a possibly broken inversion symmetry, thus fulfilling the necessary conditions for a WKSM state.

cond-mat.str-el↗

arXiv2019

Elastic properties of few unit cell thick superconducting crystals of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We present systematic measurements of the mechanical properties of few unit cell (UC) thick exfoliated crystals of a high-T$_c$ cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We determine the elastic properties of these crystals by deformation using an atomic force microscope (AFM) at room temperature. With the spatial measurements of local compliance and their detailed modeling, we independently determine the Young's modulus of rigidity and the pre-stress. The Young's modulus of rigidity is found to be in the range of 22 GPa to 30 GPa for flakes with thickness from 5 UC to 18 UC. The pre-stress spreads over the range of 5 MPa - 46 MPa, indicating a run-to-run variation during the exfoliation process. The determination of Young's modulus of rigidity for thin flakes is further verified from recently reported buckling technique.

cond-mat.mes-hall↗

arXiv2019

Nanoelectromechanical resonators from high-T$_c$ superconducting crystals of Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+δ}$

In this report, we present nanoelectromechanical resonators fabricated with thin exfoliated crystals of a high-T$_c$ cuprate superconductor Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+δ}$. The mechanical readout is performed by capacitively coupling their motion to a coplanar waveguide microwave cavity fabricated with a superconducting alloy of molybdenum-rhenium. We demonstrate mechanical frequency tunability with external dc-bias voltage, and quality factors up to 36600. Our spectroscopic and time-domain measurements show that mechanical dissipation in these systems is limited by the contact resistance arising from resistive outer layers. The temperature dependence of dissipation indicates the presence of tunneling states, further suggesting that their intrinsic performance could be as good as other two-dimensional atomic crystals such as graphene.

cond-mat.mes-hall↗

arXiv2018

An experimental and Ab-initio study of Electronic and Magnetic properties of FeGa3

Electronic structure of FeGa3 has been studied using experiments and ab-initio calculations. Magnetization measurements show that FeGa3 is inherently diamagnetic in nature. Our studies indicate that the previously reported magnetic moment on the Fe atoms in FeGa3 is not an intrinsic property of FeGa3, but is primarily due to the presence of disorder, defects, grain boundaries etc that break the symmetry about the Fe dimers. Analysis of the results obtained from magnetic measurements, photoelectron spectroscopy, Fe K-edge X-ray absorption near edge spectroscopy and ab-initio calculations clearly indicates that, the effects of on-site Coulomb repulsion between the Fe 3d electrons do not play any role in determining the electronic and magnetic properties of FeGa3. Detailed analysis of results of single crystal and poycrystalline FeGa3, helps to resolve the discrepancy in the electronic and magnetic properties in FeGa3 existing in the literature, consistently.

cond-mat.mtrl-sci↗

arXiv2017

High pressure studies on properties of FeGa3: role of on-site coulomb correlation

High pressure X-ray diffraction measurements have been carried out on the intermetallic semiconductor FeGa$_3$ and the equation of state for FeGa$_3$ has been determined. First principles based DFT calculations within the GGA approximation indicate that although the unit cell volume matches well with the experimentally obtained value at ambient pressure, it is significantly underestimated at high pressures and the difference between them increases as pressure increases. GGA + U calculations with increasing values of U$_{Fe(3d)}$ (on-site Coulomb repulsion between the Fe 3d electrons) at high pressures, correct this discrepancy. Further, the GGA+U calculations also show that along with U$_{Fe(3d)}$, the Fe 3d band width also increases with pressure and around a pressure of 4 GPa, a small density of states appear at the Fermi level. High pressure resistance measurements carried out on FeGa$_3$ also clearly show a signature of an electronic transition. Beyond the pressure of 19.7 GPa, the diffraction peaks reduce in intensity and are not observable beyond $\sim$ 26 GPa, leading to an amorphous state.

cond-mat.mtrl-sci↗

arXiv2016

Disorder-induced two-step melting of vortex matter in Co-intercalated NbSe$_2$ single crystals

Disorder induced melting, where the increase in positional entropy created by random pinning sites drives the order-disorder transition in a periodic solid, provides an alternate route to the more conventional thermal melting. Here, using real space imaging of the vortex lattice through scanning tunneling spectroscopy, we show that in the presence of weak pinning, the vortex lattice in a type II superconductor disorders through two distinct topological transitions. Across each transition, we separately identify metastable states formed through superheating of the low temperature state or supercooling of the high temperature state. Comparing crystals with different levels of pinning we conclude that the two-step melting is fundamentally associated with the presence of random pinning which generates topological defects in the ordered vortex lattice.

cond-mat.supr-con↗

arXiv2016

Orientational coupling between the vortex lattice and the crystalline lattice in a weakly pinned Co0.0075NbSe2 single crystal

We report experimental evidence of strong orientational coupling between the crystal lattice and the vortex lattice in a weakly pinned Co-doped NbSe2 single crystal through direct imaging using low temperature scanning tunneling microscopy/spectroscopy. At low fields, when the magnetic field is applied along the six-fold symmetric c-axis of the NbSe2 crystal, the equilibrium configuration of the vortex lattice is preferentially aligned along the basis vectors of the crystal lattice. The orientational coupling between the vortex lattice and crystal lattice becomes more pronounced as the magnetic field is increased. We show that this coupling enhances the stability of the orientational order of the vortex lattice, which persists even in the disordered state at high fields where dislocations and disclinations have destroyed the topological order.

cond-mat.supr-con↗

arXiv2015

Layered transition metal dichalcogenides: promising near-lattice-matched substrates for GaN growth

Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or silicon due to the extreme difficulty of obtaining a native GaN substrate. We show that several layered transition-metal dichalcogenides are closely lattice matched to GaN and report the growth of GaN on a range of such layered materials. We report detailed studies of the growth of GaN on mechanically-exfoliated flakes WS$_2$ and MoS$_2$ by metalorganic vapour phase epitaxy. Structural and optical characterization show that strain-free, single-crystal islands of GaN are obtained on the underlying chalcogenide flakes. We obtain strong near-band-edge emission from these layers, and analyse their temperature-dependent photoluminescence properties. We also report a proof-of-concept demonstration of large-area epitaxial growth of GaN on CVD MoS$_2$. Our results show that the transition-metal dichalcogenides can serve as novel near-lattice-matched substrates for nitride growth.

cond-mat.mtrl-sci↗

arXiv2015

Real space imaging of metastable Bragg glass states in a weakly pinned Type II superconductor

The breakdown of crystalline order in a disordered background connects to some of the most challenging problems in condensed matter physics. For a superconducting vortex lattice, the equilibrium state in the presence of impurities is predicted to be a "Bragg glass" (BG), where the local crystalline order is maintained everywhere and yet the global positional order decays algebraically. Here, using scanning tunnelling spectroscopy (STS) we image the vortex lattice in a weakly pinned NbSe2 single crystal. We present direct evidence that the ordered state of the VL is a BG, consisting of a large number of degenerate metastable states, which is a hallmark of a glassy state. These results are a significant step towards understanding the disordering of a lattice under the influence of quenched random disorder with a direct impact on various fields, including charge density waves, colloidal crystals and self-organised periodic structures on a substrate.

cond-mat.supr-con↗

arXiv2015

Two step disordering of the vortex lattice across the peak effect in a weakly pinned Type II superconductor, Co0.0075NbSe2

The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that at low temperatures, the vortex lattice in a 3-dimensional superconductor disorders in two steps across the peak effect. At the onset of the peak effect, the equilibrium Bragg glass transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.

cond-mat.supr-con↗