arXiv subjects

A. Thamizhavel

Publications and source records attributed to A. Thamizhavel.

At least 109 records · Page 6Linked to original sources

arXiv2014

Dual role of an ac driving force and the underlying two distinct order-disorder transitions in the vortex phase diagram of Ca3Ir4Sn13

We present distinct demarcation of the Bragg glass (BG) to multi-domain vortex glass (VG) transition line and the eventual amorphization of the VG phase in a weakly pinned single crystal of the superconducting compound Ca3Ir4Sn13 on the basis of comprehension of the different yields about the second magnetization peak (SMP) anomaly in the dc magnetization and the corresponding anomalous feature in the ac susceptibility measurements. The shaking by a small ac magnetic field, inevitably present in the ac susceptibility measurements, is seen to result in contrasting responses in two different portions of the field-temperature (H, T) phase space of the multi-domain VG. In one of the portions, embracing the BG to VG transition across the onset of the SMP anomaly, the ac drive is surprisingly seen to assist the transformation of the well ordered BG phase to a lesser ordered VG phase. The BG phase exists as a superheated state over a small portion of the VG space and this attests to the first order nature of the BG to VG transition.

cond-mat.supr-con↗

arXiv2014

Estimate of the Coulomb Correlation Energy in CeAg$_2$Ge$_2$ from Inverse Photoemission and High Resolution Photoemission Spectroscopy

The occupied and the unoccupied electronic structure of CeAg$_2$Ge$_2$ single crystal has been studied using high resolution photoemission and inverse photoemission spectroscopy respectively. High resolution photoemission reveals the clear signature of Ce $4f$ states in the occupied electronic structure which was not observed earlier due to the poor resolution. The coulomb correlation energy in this system has been determined experimentally from the position of the $4f$ states above and below the Fermi level. Theoretically the correlation energy has been determined by using the first principles density functional calculations within the generalized gradient approximations taking into account the strong intra-atomic (on-site) interaction Hubbard $U_{eff}$ term. Although the valence band calculated with different $U_{eff}$ does not show significant difference, but the substantial changes are observed in the conduction band. The estimated value of correlation energy from both the theory and the experiment is $\approx$4.2~eV for CeAg$_2$Ge$_2$.

cond-mat.mtrl-sci↗

arXiv2014

$μ$SR and inelastic neutron scattering investigations of the noncentrosymmetric antiferromagnet CeNiC$_2$

The magnetic state of the noncentrosymmetric antiferromagnet CeNiC$_2$ has been studied by magnetic susceptibility, heat capacity, muon spin relaxation ($μ$SR) and inelastic neutron scattering (INS) measurements. CeNiC$_2$ exhibits three magnetic phase transitions at $T_{N_1}$ = 20 K, $T_{N_2}$ = 10 K and $T_{N_3}$ = 2.5 K. The presence of long range magnetic order below 20 K is confirmed by the observation of oscillations in the $μ$SR spectra between 10 and 20 K and a sharp increase in the muon depolarization rate. INS studies reveal two well-defined crystal electric field (CEF) excitations around 8 and 30 meV. INS data have been analyzed using a CEF model and the wave functions were evaluated. We also calculated the direction and magnitude of the ground state moment using CEF wave functions and compare the results with that proposed from the neutron diffraction. Our CEF model correctly predicts that the moments order along the $b-$axis (or $y$-axis) and the observed magnetic moment is 0.687(5) $μ_B$, which is higher than the moment observed from the neutron diffraction (0.25 $μ_B$/Ce). We attribute the observed reduced moment due to the Kondo screening effect.

cond-mat.str-el↗

arXiv2014

Probing anisotropy in a new noncentrosymmetric superconductor BiPd

Strength of Rashba type antisymmetric spin orbit coupling has the crucial effect on superconducting and normal state properties in noncentrosymmetric (NCS) superconductors. This can be seen on various anisotropic properties of the system. For a system with lowered crystalline symmetry as in noncentrosymmetric crystals, the superconducting pairing state can have correlation to the normal state properties. In our earlier work we have shown that alfa-BiPd which has a monoclinic (P_21) structure with the absence of the centre of inversion exhibits bulk superconductivity at 3.8 K. In order to understand the role of spin-orbit scattering, we have studied the anisotropic properties of high quality single crystal (RRR 170) of NCS BiPd by measuring magnetic susceptibility, electrical transport from 1.8 K to 300 K and isothermal M_H loop (1.8 K to 3.8 K) in the superconducting state for <010> and <100> crystallographic directions respectively. Moderate anisotropy up to 30 % has been found in the electrical transport possibly indicating significant Rashba type antisymmetric spin orbit coupling present in alfa-BiPd.

cond-mat.supr-con↗

arXiv2014

Anisotropic superconductivity in the noncentrosymmetric BiPd

We report measurements of London penetration depth $λ(T)$ for the noncentrosymmetric superconductor BiPd by using a tunnel diode oscillator. Pronounced anisotropic behavior is observed in the low-temperature penetration depth; the in-plane penetration depth $λ_{ac}(T)$ follows an exponential decrease, but the interplane penetration depth $λ_b(T)$ shows power-law-type behavior. The superfluid density $ρ_s(T)$, converted from the penetration depth $λ(T)$, is best fitted by an anisotropic two-band BCS model. We argue that such a complex order parameter is attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of antisymmetric spin-orbit coupling in BiPd.

cond-mat.supr-con↗

arXiv2014

EuNiGe3, an equal moment anisotropic antiferromagnet

Single crystals of EuNiGe$_3$ crystallizing in the non-centrosymmetric BaNiSn$_3$-type structure have been grown using In flux, enabling us to explore the anisotropic magnetic properties which was not possible with previously reported polycrystalline samples. The EuNiGe$_3$ single crystalline sample is found to order antiferromagnetically at 13.2 K as revealed from the magnetic susceptibility, heat capacity and electrical resistivity data. The low temperature magnetization M(H) is distinctly different for field parallel to ${ab}$-plane and $c$-axis; the ${ab}$-plane magnetization varies nearly linearly with field before the occurrence of an induced ferromagnetic phase (spin-flip) at 6.2 Tesla; on the other hand M(H) along the $c$-axis is accompanied by two metamagnetic transitions followed by a spin-flip at 4.1 T. A model including anisotropic exchange and dipole-dipole interactions reproduces the main features of magnetization plots but falls short of full representation. (H,T) phase diagrams have been constructed for the field applied along the principal directions. From the $^{151}$Eu Mössbauer spectra, we determine that the 13.2 K transition leads to an incommensurate antiferromagnetic intermediate phase followed by a transition near 10.5 K to a commensurate antiferromagnetic configuration.

cond-mat.str-el↗

arXiv2014

Anisotropic magnetic properties and giant magnetocaloric effect of PrSi single crystal

Single crystal of PrSi was grown by Czochralski method in a tetra-arc furnace. Powder x-ray diffraction of the as grown crystal revealed that PrSi crystallizes in FeB$-$type structure with space group $Pnma$ (no. 62). PrSi undergoes a ferromagnetic transition at 52 K with [010] direction as the easy axis of magnetization. Heat capacity data confirm the bulk nature of the transition at 52 K and exhibit a huge anomaly at the transition. A sharp rise in the low temperature heat capacity has been observed (below 5 K) which is attributed to the $^{141}$Pr nuclear Schottky heat capacity arising from the hyperfine field of the Pr moment. The estimated Pr magnetic moment 2.88 $μ_{\rm B}$/Pr from the hyperfine splitting is in agreement with the saturation magnetization value obtained from the magnetization data measured at 2 K. From the crystal electric field (CEF) analysis of the magnetic susceptibility, magnetization and the heat capacity data it is found that the degenerate $J = 4$ Hund's rule derived state of Pr$^{3+}$-ion splits into nine singlets with an overall splitting of 284 K, the first excited singlet state separated by just 9 K from the ground state. The magnetic ordering in PrGe appears to be due to the exchange generated admixture of low lying crystal field levels. Magnetocaloric effect (MCE) has been investigated from magnetization data along all the three principal crystallographic directions. Large magnetic entropy change, $-ΔS_M = $22.2 J/kg K, and the relative cooling power, RCP = $460$ J/kg, characteristic of giant magneto caloric effect are achieved near the transition temperature ($T_{\rm C}$ = 52 K) for $H =$~70 kOe along $[010]$. Furthermore, the PrSi single crystal exhibits a giant MCE anisotropy.

cond-mat.str-el↗

arXiv2013

Electronic structure of CaFe2As2

We investigate the electronic structure of CaFe$_2$As$_2$ using high resolution photoemission spectroscopy. Experimental results exhibit three energy bands crossing the Fermi level making hole pockets around the $Γ$-point. Temperature variation reveal a gradual shift of an energy band away from the Fermi level with the decrease in temperature in addition to the spin density wave (SDW) transition induced Fermi surface reconstruction of the second energy band across SDW transition temperature. The hole pocket in the former case eventually disappears at lower temperatures while the hole Fermi surface of the third energy band possessing finite $p$ orbital character survives till the lowest temperature studied. These results reveal signature of a complex charge redistribution among various energy bands as a function of temperature that might be associated to the exotic properties of this system.

cond-mat.supr-con↗

arXiv2013

Magnetic properties of single crystalline CeMg$_{12}$

Single crystal of CeMg$_{12}$ is obtained by Bridgman method. CeMg$_{12}$ crystallizes in the tetragonal structure with space group $I4/mmm$ (#139). The Laue pattern confirms the tetragonal crystal structure of CeMg$_{12}$. We have studied the magnetic properties by measuring magnetic susceptibility, magnetization, heat capacity and electrical transport. Specific heat measurement shows that the compound orders at 1.2 K. We have measured the magnetic susceptibility in the temperature range 1.8 to 300 K, and the susceptibility data show that [100] crystallographic direction is the easy axis of magnetization. At high temperature inverse susceptibility varies linearly with temperature, and follows the Curie-Weiss behaviour. The effective moment is close to the free ion value thus indicating Ce is in trivalent state in this compound.

cond-mat.str-el↗

arXiv2013

Physical, optical and nonlinear properties of InS single crystal

Indium Sulphide (InS) single crystals are successfully grown by In flux. Single crystal X-ray diffraction shows orthorhombic structure of Pnnm space group. Ellipsometry measurements performed on the (010) oriented crystal exhibit low anisotropy in the 300-1000 nm wavelength range and consequently negligible THz emission is observed. Optical band gap of $2.09 eV$ is deduced from linear optical measurements. Nonlinear optical properties are studied by single beam Z-scan measurements at 800 nm, where two-photon absorption is present. Nonlinear refractive index and absorption coefficient are estimated to be $η_2$ = $2.3 10^{-11} cm^2/W$ and $β$= $62.4 cm/ GW$, respectively for excitation intensity of $0.32 GW/cm^2$. The origin of nonlinearity in InS crystal is accounted to be due to the third-order anharmonic motion of the bound electrons.

cond-mat.mtrl-sci↗

arXiv2013

Anisotropic magnetic properties and crystal electric field studies on CePd$_2$Ge$_2$ single crystal

The anisotropic magnetic properties of the antiferromagnetic compound CePd$_2$Ge$_2$, crystallizing in the tetragonal crystal structure have been investigated in detail on a single crystal grown by Czochralski method. From the electrical transport, magnetization and heat capacity data, the Néel temperature is confirmed to be 5.1 K. Anisotropic behaviour of magnetization and resistivity is observed along the two principal crystallographic directions viz., [100] and [001]. The isothermal magnetization measured in the magnetically ordered state at 2 K exhibits a spin re-orientation at 13.5 T for field applied along [100] direction, whereas the magnetization was linear along the [001] direction attaining a value of 0.94 $μ_{\rm B}$/Ce at 14 T. The reduced value of the magnetization is attributed to the crystalline electric field (CEF) effects. A sharp jump in the specific heat at the magnetic ordering temperature is observed. After subtracting the phononic contribution, the jump in the heat capacity amounts to 12.5 J/K mol which is the expected value for a spin ${1}{2}$ system. From the CEF analysis of the magnetization data the excited crystal field split energy levels were estimated to be at 120 K and 230 K respectively, which quantitatively explain the observed Schottky anomaly in the heat capacity. A magnetic phase diagram has been constructed based on the field dependence of magnetic susceptibility and the heat capacity data.

cond-mat.str-el↗

arXiv2013

Multiple charge density wave transitions in Lu_{2} Ir_{3} Si_{5} single crystal

The physical properties of the single-crystalline samples of Lu_{2} Ir_{3} Si_{5} have been investigated by magnetic susceptibility, resistivity and heat capacity studies. We observed multiple charge density wave (CDW) transitions in all the measurements. A strong thermal hysteresis at these transitions suggests a possible first order CDW ordering. In addition, the first order nature is ascertained by a very narrow and a huge cusp (62 J/mol K) in the zero field specific heat data which also suggests a strong interchain coupling. By applying a field of 9T in the specific heat measurement, one of the CDW transitions is suppressed.

cond-mat.str-el↗

arXiv2013

Superconductivity in a low carrier density system: A single crystal study of cubic Y$_3$Ru$_4$Ge$_{13}$

We have successfully grown the single crystal of a low carrier density system Y$_3$Ru$_4$Ge$_{13}$ which crystallizes in the cubic crystal structure with the space group Pm3n. Y$_3$Ru$_4$Ge$_{13}$ exhibits super conductivity below 2.85 K as determined from the electrical resistivity, magnetic susceptibility and heat capacity measurements. The bulk measurements indicate multiband superconductivity in this low carrier system. Hall effect measurements show that the Hall coefficient R$_H$ is positive and it decreases rapidly with temperature which is highly unusual. The simple estimation of the carrier concentration gives a value (\sim 10$^{20}$/cm$^3$) which is 2-3 orders of magnitude lower than that of a conventional metal like Cu which seems to suggest that Y$_3$Ru$_4$Ge$_{13}$ is a semimetal that display multiband superconductivity.

cond-mat.supr-con↗

arXiv2012

Direct evidence of magnetic correlations deep inside the superconducting state of CaFe1.94Co0.06As2

Imaging local magnetic field distribution in a single crystal of superconductor CaFe1.94Co0.06As2 shows the presence of anomalous remnant magnetization within Meissner like regions of the sample. Close to the superconducting transition temperature we find a coexistence of superconductivity and magnetic correlations. While the magnetic correlations are enhanced with the lowering of temperature, interestingly the area over which the magnetic phase exists shrinks at the expense of a growing superconducting phase in the sample. The coexistence of two phases deep inside the superconducting phase in this compound is maintained within the temperature range that we have explored in our experiment.

cond-mat.supr-con↗

arXiv2012

Anomalous local magnetic field distribution and strong pinning in CaFe1.94Co0.06As2 single crystals

Magneto-optical imaging of a single crystal of CaFe1.94Co0.06As2, shows anomalous remnant magnetization within Meissner like regions of the superconductor. The unconventional shape of the local magnetization hysteresis loop suggests admixture of superconducting and magnetic fractions governing the response. Near the superconducting transition temperature, local magnetic field exceeds the applied field resulting in a diamagnetic to positive magnetization transformation. The observed anomalies in the local magnetic field distribution are accompanied with enhanced bulk pinning in the CaFe1.94Co0.06As2 single crystals. We propose our results suggest a coexistence of superconductivity and magnetic correlations.

cond-mat.supr-con↗

arXiv2012

Antiferro and ferromagnetic ordering in PrGe single crystal

The equiatomic PrGe single crystal was grown by Czochralski pulling method. The grown single crystal was found to have CrB-type orthorhombic crystal structure with the space group \textit{Cmcm} (#63). Transport and magnetization data reveal large anisotropy in the electrical resistivity, magnetic susceptibility and magnetization. PrGe was found to exhibit two consecutive magnetic orderings at 44 K and 41.5 K, respectively. The magnetic susceptibility measurement along the three principal directions, in low applied fields, revealed a cusp like behaviour at 44 K while at 41.5 K a ferromagnetic like increase was observed. The hysteritic behaviour in the magnetization measurement at 1.8 K confirmed the ferromagnetic nature of PrGe at low temperatures. The heat capacity data clearly revealed the bulk nature of two magnetic transitions by the presence of two sharp peaks attaining values exceeding 40 J/K$\cdot$mol at the respective temperatures. The absence of Schottky contribution in the magnetic part of heat capacity indicates a quasi-ninefold degenerate J=4 magnetic ground state in this system. The low temperature data of electrical resistivity and the magnetic part of heat capacity show an existence of gap in the spin-wave spectrum.

cond-mat.str-el↗

arXiv2012

Magnetic and transport properties of Pr2Pt3Si5

We have investigated the magnetic and transport properties of a polycrystalline Pr2Pt3Si5 sample through the dc and ac magnetic susceptibilities, electrical resistivity, and specific heat measurements. The Rietveld refinement of the powder X-ray diffraction data reveals that Pr2Pt3Si5 crystallizes in the U2Co3Si5-type orthorhombic structure (space group Ibam). Both the dc and ac magnetic susceptibility data measured at low fields exhibit sharp anomaly near 15 K. In contrast, the specific heat data exhibit only a broad anomaly implying no long range magnetic order down to 2 K. The broad Schottky-type anomaly in low temperature specific heat data is interpreted in terms of crystal electric field (CEF) effect, and a CEF-split singlet ground state is inferred. The absence of the long range order is attributed to the presence of nonmagnetic singlet ground state of the Pr3+ ion. The electrical resistivity data exhibit metallic behavior and are well described by the Bloch-Grüniesen-Mott relation.

cond-mat.str-el↗

arXiv2012

Field-induced quantum fluctuations in the heavy fermion superconductor CeCu2Ge2

Quantum-mechanical fluctuations in strongly correlated electron systems cause unconventional phenomena such as non-Fermi liquid behavior, and arguably high temperature superconductivity. Here we report the discovery of a field-tuned quantum critical phenomenon in stoichiometric CeCu2Ge2, a spin density wave ordered heavy fermion metal that exhibits unconventional superconductivity under ~ 10 GPa of applied pressure. Our finding of the associated quantum critical spin fluctuations of the antiferromagnetic spin density wave order, dominating the local fluctuations due to single-site Kondo effect, provide new information about the underlying mechanism that can be important in understanding superconductivity in this novel compound.

cond-mat.str-el↗