arXiv subjects

C. Petrovic

Publications and source records attributed to C. Petrovic.

At least 91 records · Page 5Linked to original sources

arXiv2016

Superconducting Order from Disorder in 2H-TaSe$_{2-x}$S$_{x}$ (0$\leq$x$\leq$2)

We report on the emergence of robust superconducting order in single crystal alloys of 2H-TaSe$_{2-x}$S$_{x}$ (0$\leq$x$\leq$2) . The critical temperature of the alloy is surprisingly higher than that of the two end compounds TaSe$_{2}$ and TaS$_{2}$. The evolution of superconducting critical temperature T$_{c} (x)$ correlates with the full width at half maximum of the Bragg peaks and with the linear term of the high temperature resistivity. The conductivity of the crystals near the middle of the alloy series is higher or similar than that of either one of the end members 2H-TaSe$_{2}$ and/or 2H-TaS$_{2}$. It is known that in these materials superconductivity (SC) is in close competition with charge density wave (CDW) order. We interpret our experimental findings in a picture where disorder tilts this balance in favor of superconductivity by destroying the CDW order.

cond-mat.supr-con↗

arXiv2016

Raman spectroscopy of K_xCo_{2-y}Se_2 single crystals near the ferromagnet-paramagnet transition

Polarized Raman scattering spectra of the K_xCo_{2-y}Se_2 single crystals reveal the presence of two phonon modes, assigned as of the A_{1g} and B_{1g} symmetry. Absence of additional modes excludes the possibility of vacancy ordering, unlike in K_xFe_{2-y}Se_2. The ferromagnetic (FM) phase transition at T_c\approx 74 K leaves a clear fingerprint on the temperature dependence of the Raman mode energy and linewidth. For T>T_c the temperature dependence looks conventional, driven by the thermal expansion and anharmonicity. The Raman modes are rather broad due to the electron-phonon coupling increased by the disorder and spin fluctuation effects. In the FM phase the phonon frequency of both modes increases, while an opposite trend is seen in their linewidth: the A_{1g} mode narrows in the FM phase, whereas the B_{1g} mode broadens. We argue that the large asymmetry and anomalous frequency shift of the B_{1g} mode is due to the coupling of spin fluctuations and vibration. Our density functional theory (DFT) calculations for the phonon frequencies agree rather well with the Raman measurements, with some discrepancy being expected since the DFT calculations neglect the spin fluctuations.

cond-mat.mtrl-sci↗

arXiv2016

Fano q-reversal in topological insulator Bi2Se3

We studied magneto-optical response of a canonical topological insulator Bi$_2$Se$_3$ with the goal of addressing a controversial issue of electron-phonon coupling. Magnetic-field induced modifications of reflectance are very pronounced in the infrared part of the spectrum, indicating strong electron-phonon coupling. This coupling causes an asymmetric line-shape of the 60 cm$^{-1}$ phonon mode, and is analyzed within the Fano formalism. The analysis reveals that the Fano asymmetry parameter (q) changes sign when the cyclotron resonance is degenerate with the phonon mode. To the best of our knowledge this is the first example of magnetic field driven q-reversal.

cond-mat.str-el↗

arXiv2016

Multiband nodeless superconductivity near the charge-density-wave quantum critical point in ZrTe3-xSex

Recently it was found that selenium doping can suppress the charge-density-wave (CDW) order and induce bulk superconductivity in ZrTe$_3$. The observed superconducting dome suggests the existence of a CDW quantum critical point (QCP) in ZrTe$_{3-x}$Se$_x$ near $x \approx$ 0.04. To elucidate its superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe$_{3-x}$Se$_x$ single crystals ($x$ = 0.044 and 0.051) down to 80 mK. For both samples, the residual linear term $κ_0/T$ at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of $κ_0/T$ manifests multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe$_{3-x}$Se$_x$, which indicates conventional superconductivity despite of the existence of a CDW QCP.

cond-mat.supr-con↗

arXiv2016

Evidence of superconductivity-induced phonon spectra renormalization in alkali-doped iron selenides

Polarized Raman scattering spectra of superconducting K$_x$Fe$_{2-y}$Se$_2$ and nonsuperconducting K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$ single crystals were measured in a temperature range from 10 K up to 300 K. Two Raman active modes from the $I4/mmm$ phase and seven from the $I4/m$ phase are observed in frequency range from 150 to 325 cm$^{-1}$ in both compounds, suggesting that K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$ single crystal also has two-phase nature. Temperature dependence of Raman mode energy is analyzed in terms of lattice thermal expansion and phonon-phonon interaction. Temperature dependence of Raman mode linewidth is dominated by temperature-induced anharmonic effects. It is shown that change of Raman mode energy with temperature is dominantly driven by thermal expansion of the crystal lattice. Abrupt change of the A$_{1g}$ mode energy near $T_C$ was observed in K$_x$Fe$_{2-y}$Se$_2$, whereas it is absent in non-superconducting K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$. Phonon energy hardening at low temperatures in the superconducting sample is a consequence of superconductivity-induced redistribution of the electronic states below critical temperature.

cond-mat.mtrl-sci↗

arXiv2016

Lattice dynamics of KNi2Se2

We report the first principle calculations of the lattice dynamics of KNi$_{2}$Se$_2$ together with the Raman scattering study.\ We have observed three out of four Raman active modes predicted by the factor group analysis. Calculated phonon frequencies are in good agreement with experimental findings.\ Contrary to its iron counterpart (K$_x$Fe$_{2-y}$Se$_2$), K$_{0.95}$Ni$_{1.86}$Se$_2$ does not show vacancy ordering.

cond-mat.supr-con↗

arXiv2016

Probing IrTe2 crystal symmetry by polarized Raman scattering

Polarized Raman scattering measurements on IrTe$_2$ single crystals carried out over 15~K - 640~K temperature range, and across the structural phase transition, reveal new insights regarding the crystal symmetry.\ In the high temperature regime three Raman active modes are observed at all studied temperatures above the structural phase transition, rather than two as predicted by the factor group analysis for the assumed $P\bar{3}m1$ symmetry. This indicates that the actual symmetry of the high temperature phase is lower than previously thought.\ Observation of an additional E$_g$ mode at high temperature can be explained by doubling of the original trigonal unit cell along the $c$-axis and within the $P\bar{3}c1$ symmetry.\ In the low temperature regime (below 245 K) the new Raman modes appear as a consequence of the symmetry lowering phase transition and corresponding increase of the primitive cell.\ All the modes observed below the phase transition temperature can be assigned within the monoclinic crystal symmetry.\ Temperature dependence of the Raman active phonons in both phases are mainly driven by anharmonicity effects. The results call for reconsideration of the crystallographic phases of IrTe$_2$.

cond-mat.supr-con↗

arXiv2016

Interlayer electronic transport in CaMnBi$_{2}$ antiferromagnet

We report interlayer electronic transport in CaMnBi$_{2}$ single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. Small cyclotron mass, large mobility of carriers and nontrivial Berry's phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface. Similar to SrMnBi$_{2}$ that features an anisotropic Dirac cone, our results suggest that magnetic field-induced changes in the interlayer conduction are also present in layered bismuth-based materials with zero-energy line in momentum space created by the staggered alkaline earth atoms.

cond-mat.mes-hall↗

arXiv2016

Critical current density and vortex pinning in tetragonal FeS$_{1-x}$Se$_{x}$ ($x=0,0.06$)

We report critical current density ($J_c$) in tetragonal FeS single crystals, similar to iron based superconductors with much higher superconducting critical temperatures ($T_{c}$'s). The $J_c$ is enhanced 3 times by 6\% Se doping. We observe scaling of the normalized vortex pinning force as a function of reduced field at all temperatures. Vortex pinning in FeS and FeS$_{0.94}$Se$_{0.06}$ shows contribution of core-normal surface-like pinning. Reduced temperature dependence of $J_c$ indicates that dominant interaction of vortex cores and pinning centers is via scattering of charge carriers with reduced mean free path ($δ$$l$), in contrast to K$_x$Fe$_{2-y}$Se$_2$ where spatial variations in $T_{c}$ ($δ$$T_{c}$) prevails.

cond-mat.supr-con↗

arXiv2016

Multiband electronic transport in $α$-Yb$_{1-x}$Sr$_{x}$AlB$_{4}$ [x = 0, 0.19(3)] single crystals

We report on the evidence for the multiband electronic transport in $α$-YbAlB$_{4}$ and $α$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Multiband transport reveals itself below 10 K in both compounds via Hall effect measurements, whereas anisotropic magnetic ground state sets in below 3 K in $α$-Yb$_{0.81(2)}$Sr$_{0.19(3)}$AlB$_{4}$. Our results show that Sr$^{2+}$ substitution enhances conductivity, but does not change the quasiparticle mass of bands induced by heavy fermion hybridization.

cond-mat.str-el↗

arXiv2016

Electrodynamic response of type II Weyl semimetals

Weyl fermions play a major role in quantum field theory but have been quite elusive as fundamental particles. Materials based on quasi two-dimensional bismuth layers were recently designed and provide an arena for the study of the interplay between anisotropic Dirac fermions, magnetism and structural changes, allowing the formation of Weyl fermions in condensed matter. Here, we perform an optical investigation of YbMnBi$_2$, a representative type II Weyl semimetal, and contrast its excitation spectrum with the optical response of the more conventional semimetal EuMnBi$_2$. Our comparative study allows us disentangling the optical fingerprints of type II Weyl fermions, but also challenge the present theoretical understanding of their electrodynamic response.

cond-mat.str-el↗

arXiv2016

Quantum critical quasiparticle scattering within the superconducting state of CeCoIn5

The thermal conductivity kappa of the heavy-fermion metal CeCoIn5 was measured in the normal and superconducting states as a function of temperature T and magnetic field H, for a current and field parallel to the [100] direction. Inside the superconducting state, when the field is lower than the upper critical field Hc2, kappa/T is found to increase as T approaches absolute zero, just as in a metal and in contrast to the behavior of all known superconductors. This is due to unpaired electrons on part of the Fermi surface, which dominate the transport above a certain field. The evolution of kappa/T with field reveals that the electron-electron scattering (or transport mass m^*) of those unpaired electrons diverges as H approaches Hc2 from below, in the same way that it does in the normal state as H approaches Hc2 from above. This shows that the unpaired electrons sense the proximity of the field-tuned quantum critical point of CeCoIn5 at H^* = Hc2 even from inside the superconducting state. The fact that the quantum critical scattering of the unpaired electrons is much weaker than the average scattering of all electrons in the normal state reveals a k-space correlation between the strength of pairing and the strength of scattering, pointing to a common mechanism, presumably antiferromagnetic fluctuations.

cond-mat.str-el↗

arXiv2016

Enhancement of the thermoelectric properties in doped FeSb$_2$ bulk crystals

Kondo insulator FeSb$_2$ with large Seebeck coefficient would have potential in thermoelectric applications in cryogenic temperature range if it had not been for large thermal conductivity $κ$. Here we studied the influence of different chemical substitutions at Fe and Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. At $5\%$ of Te doping at Sb site thermal conductivity is suppressed from $\sim 250$ W/Km in undoped sample to about 8 W/Km. However, Cr and Co doping at Fe site suppresses thermal conductivity more slowly than Te doping, and even at 20$\%$ Cr/Co doping the thermal conductivity remains $\sim 30$ W/Km. The analysis of different contributions to phonon scattering indicates that the giant suppression of $κ$ with Te is due to the enhanced point defect scattering originating from the strain field fluctuations. In contrast, Te-doping has small influence on the correlation effects and then for small Te substitution the large magnitude of the Seebeck coefficient is still preserved, leading to the enhanced thermoelectric figure of merit ($ZT\sim 0.05$ at $\sim 100$ K) in Fe(Sb$_{0.9}$Te$_{0.1}$)$_2$.

cond-mat.str-el↗

arXiv2016

Ca$_3$Ir$_4$Sn$_{13}$: A weakly correlated nodeless superconductor

We report detailed Seebeck coefficient, Hall resistivity as well as specific heat measurement on Ca$_3$Ir$_4$Sn$_{13}$ single crystals. The Seebeck coefficient exhibits a peak corresponding to the anomaly in resistivity at $T^*$, and the carrier density is suppressed significantly below $T^*$. This indicates a significant Fermi surface reconstruction and the opening of the charge density wave gap at the supperlattice transition. The magnetic field induced enhancement of the residual specific heat coefficient $γ(H)$ exhibits a nearly linear dependence on magnetic field, indicating a nodeless gap. In the temperature range close to $T_c$ the Seebeck coefficient can be described well by the diffusion model. The zero-temperature extrapolated thermoelectric power is very small, implying large normalized Fermi temperature. Consequently the ratio $\frac{T_c}{T_F}$ is very small. Our results indicate that Ca$_3$Ir$_4$Sn$_{13}$ is a weakly correlated nodeless superconductor.

cond-mat.supr-con↗

arXiv2016

Multiband effects and the possible Dirac states in LaAgSb$_2$

Here we report the possible signature of Dirac fermions in the magnetoresistance, Hall resistivity and magnetothermopower of LaAgSb$_2$. The opposite sign between Hall resistivity and Seebeck coefficient indicates the multiband effect. Electronic structure calculation reveals the existence of the linear bands and the parabolic bands crossing the Fermi level. The large linear magnetoresistance was attributed to the quantum limit of the possible Dirac fermions or the breakdown of weak-field magnetotransport at the charge density wave phase transition. Analysis of Hall resistivity using two-band model reveals that Dirac holes which dominate the electronic transport have much higher mobility and larger density than conventional electrons. Magnetic field suppresses the apparent Hall carrier density, and also induces the sign change of the Seebeck coefficient from negative to positive. These effects are possibly attributed to the magnetic field suppression of the density of states at the Fermi level originating from the quantum limit of the possible Dirac holes.

cond-mat.mes-hall↗

arXiv2016

Multiband behavior and non-metallic low-temperature state of K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$

We report evidence for multiband transport and an insulating low-temperature normal state in superconducting K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ with $T_{c}\approx 20$ K. The temperature-dependent upper critical field, $H_{c2}$, is well described by a two-band BCS model. The normal-state resistance, accessible at low temperatures only in pulsed magnetic fields, shows an insulating logarithmic temperature dependence as $T \rightarrow 0$ after superconductivity is suppressed. This is similar as for high-$T_{c}$ copper oxides and granular type-I superconductors, suggesting that the superconductor-insulator transition observed in high magnetic fields is related to intrinsic nanoscale phase separation.

cond-mat.supr-con↗

arXiv2016

Evolution of the Pauli spin-paramagnetic effect on the upper critical fields of K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ single crystals

We have studied the temperature dependence of the upper critical fields, $μ_{0}H_{c2}$, of K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ single crystals up to 60 T. The $μ_{0}H_{c2}$ for $H\parallel ab$ and $H\parallel c$ decrease with increasing sulfur content. The detailed analysis using Werthamer-Helfand-Hohenberg (WHH) theory including the Pauli spin-paramagnetic effect shows that $μ_{0}H_{c2}$ for $H\parallel ab$ is dominated by the spin-paramagnetic effect, which diminishes with higher S content, whereas $μ_{0}H_{c2}$ for $H\parallel c$ shows a linear temperature dependence with an upturn at high fields. The latter observation can be ascribed to multiband effects that become weaker for higher S content. This results in an enhanced anisotropy of $μ_{0}H_{c2}$ for high S content due to the different trends of the spin-paramagnetic and multiband effect for $H\parallel ab$ and $H\parallel c$, respectively.

cond-mat.supr-con↗

arXiv2016

Spin glass in semiconducting KFe$_{1.05}$Ag$_{0.88}$Te$_{2}$ single crystals

We report discovery of KFe$_{1.05}$Ag$_{0.88}$Te$_{2}$ single crystals with semiconducting spin glass ground state. Composition and structure analysis suggest nearly stoichiometric I4/mmm space group but allow for the existence of vacancies, absent in long range semiconducting antiferromagnet KFe$_{0.85}$Ag$_{1.15}$Te$_{2}$. The subtle change in stoichometry in Fe/Ag sublattice changes magnetic ground state but not conductivity, giving further insight into the semiconducting gap mechanism.

cond-mat.str-el↗