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Search Results: 1 - 10 of 462054 matches for " A. Akrap "
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Hysteretic behavior at the collapse of the metal-insulator transition in BaVS$_3$
N. Bari?i?,A. Akrap,H. Berger,L. Forró
Physics , 2007,
Abstract: Electrical resistivity as a function of temperature, pressure, and magnetic field was measured in high and low purity single crystals of BaVS$_3$ close to the critical pressure value $p_{cr}$$\approx$2 GPa, associated with the zero temperature insulator-to-metal (MI) transition. In the 1.8-2.0 GPa range, where the MI transition is below $\approx$20 K, one can observe a sudden collapse of the MI phase boundary upon increasing pressure and at fixed pressure a magnetic field induced insulator to metal transition. In high quality samples these features are accompanied by hysteresis in all measured physical quantities as a function of temperature and magnetic field. We ascribe these observations to the crossing of the MI and the magnetic phase boundaries upon increasing pressure.
The evolution of the Non-Fermi Liquid behavior of BaVS$_3$ under high pressure
N. Bari?i?,A. Akrap,H. Berger,L. Forró
Physics , 2007,
Abstract: Temperature, pressure, and magnetic field dependencies of the resistivity of BaVS$_3$ were measured above the critical pressure of $p_{cr}$=2 GPa, which is associated with the zero temperature insulator-to-metal (MI) transition. The resistivity exhibits the $T^n$ temperature dependence below $T_g\approx$15 K, with $n$ of 1.5 at $p_{cr}$, which increases continuously with pressure towards 2. This is interpreted as a crossover from non-Fermi (NFL) to Fermi-liquid (FL) behavior. Although the spin configuration of the $e_g$ electrons influences the charge propagation, the NFL behavior is attributed to the pseudogap that appears in the single particle spectrum of the $d_z^2$ electrons related to large quasi-one dimensional (Q-1d) 2$k_F$-CDW fluctuations. The non-monotonic magnetic field dependence of $\Delta$$\rho$/$\rho$ reveals a characteristic field $B_0\approx$12 T attributed to the full suppression of the pseudogap.
Manifestations of fine features of the density of states in the transport properties of KOs2O6
A. Akrap,E. Tutis,S. M. Kazakov,N. D. Zhigadlo,J. Karpinski,L. Forro
Physics , 2007, DOI: 10.1103/PhysRevB.75.172501
Abstract: We performed high-pressure transport measurements on high-quality single crystals of KOs2O6, a beta-pyrochlore superconductor. While the resistivity at high temperatures might approach saturation, there is no sign of saturation at low temperatures, down to the superconducting phase. The anomalous resistivity is accompanied by a nonmetallic behavior in the thermoelectric power (TEP) up to temperatures of at least 700 K, which also exhibits a broad hump with a maximum at 60 K. The pressure influences mostly the low-energy electronic excitations. A simple band model based on enhanced density of states in a narrow window around the Fermi energy (EF) explains the main features of this unconventional behavior in the transport coefficients and its evolution under pressure.
Optical conductivity of nodal metals
C. C. Homes,J. J. Tu,J. Li,G. D. Gu,A. Akrap
Physics , 2013, DOI: 10.1038/srep03446
Abstract: Fermi liquid theory is remarkably successful in describing the transport and optical properties of metals; at frequencies higher than the scattering rate, the optical conductivity adopts the well-known power law behavior $\sigma_1(\omega) \propto \omega^{-2}$. We have observed an unusual non-Fermi liquid response $\sigma_1(\omega) \propto \omega^{-1\pm 0.2}$ in the ground states of several cuprate and iron-based materials which undergo electronic or magnetic phase transitions resulting in dramatically reduced or nodal Fermi surfaces. The identification of an inverse (or fractional) power-law behavior in the residual optical conductivity now permits the removal of this contribution, revealing the direct transitions across the gap and allowing the nature of the electron-boson coupling to be probed. The non-Fermi liquid behavior in these systems may be the result of a common Fermi surface topology of Dirac cone-like features in the electronic dispersion.
Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
T. Ivek,T. Vuletic,S. Tomic,A. Akrap,H. Berger,L. Forro
Physics , 2007, DOI: 10.1103/PhysRevB.78.035110
Abstract: The charge response in the barium vanadium sulfide (BaVS3) single crystals is characterized by dc resistivity and low frequency dielectric spectroscopy. A broad relaxation mode in MHz range with huge dielectric constant ~= 10^6 emerges at the metal-to-insulator phase transition TMI ~= 67 K, weakens with lowering temperature and eventually levels off below the magnetic transition Tchi ~= 30 K. The mean relaxation time is thermally activated in a manner similar to the dc resistivity. These features are interpreted as signatures of the collective charge excitations characteristic for the orbital ordering that gradually develops below TMI and stabilizes at long-range scale below Tchi.
Optical properties and electronic structure of the nonmetallic metal FeCrAs
A. Akrap,Y. M. Dai,W. Wu,S. R. Julian,C. C. Homes
Physics , 2013, DOI: 10.1103/PhysRevB.89.125115
Abstract: The complex optical properties of a single crystal of hexagonal FeCrAs ($T_N \simeq 125$ K) have been determined above and below $T_N$ over a wide frequency range in the planes (along the $b$ axis), and along the perpendicular ($c$ axis) direction. At room temperature, the optical conductivity $\sigma_1(\omega)$ has an anisotropic metallic character. The electronic band structure reveals two bands crossing the Fermi level, allowing the optical properties to be described by two free-carrier (Drude) contributions consisting of a strong, broad component and a weak, narrow term that describes the increase in $\sigma_1(\omega)$ below $\simeq 15$ meV. The dc-resistivity of FeCrAs is ``non-metallic'', meaning that it rises in power-law fashion with decreasing temperature, without any signature of a transport gap. In the analysis of the optical conductivity, the scattering rates for both Drude contributions track the dc-resistivity quite well, leading us to conclude that the non-metallic resistivity of FeCrAs is primarily due to a scattering rate that increases with decreasing temperature, rather than the loss of free carriers. The power law $\sigma_1(\omega) \propto \omega^{-0.6}$ is observed in the near-infrared region and as $T\rightarrow T_N$ spectral weight is transferred from low to high energy ($\gtrsim 0.6$ eV); these effects may be explained by either the two-Drude model or Hund's coupling. We also find that a low-frequency in-plane phonon mode decreases in frequency for $T < T_N$, suggesting the possibility of spin-phonon coupling.
Optical properties of BiTeBr and BiTeCl
A. Akrap,J. Teyssier,A. Magrez,P. Bugnon,H. Berger,A. B. Kuzmenko,D. van der Marel
Physics , 2014, DOI: 10.1103/PhysRevB.90.035201
Abstract: We present a comparative study of the optical properties - reflectance, transmission and optical conductivity - and Raman spectra of two layered bismuth-tellurohalides BiTeBr and BiTeCl at 300 K and 5 K, for light polarized in the a-b planes. Despite different space groups, the optical properties of the two compounds are very similar. Both materials are doped semiconductors, with the absorption edge above the optical gap which is lower in BiTeBr (0.62 eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two materials. A non-Drude free carrier contribution in the optical conductivity, as well as three Raman and two infrared phonon modes, are observed in each compound. There is a dramatic difference in the highest infrared phonon intensity for the two compounds, and a difference in the doping levels. Aspects of the strong electron-phonon interaction are identified. Several interband transitions are assigned, among them the low-lying absorption $\beta$ which has the same value 0.25 eV in both compounds, and is caused by the Rashba spin splitting of the conduction band. An additional weak transition is found in BiTeCl, caused by the lower crystal symmetry.
Phonon anomaly in BaFe2As2
A. Akrap,J. J. Tu,L. J. Li,G. H. Cao,X. A. Xu,C. C. Homes
Physics , 2009, DOI: 10.1103/PhysRevB.80.180502
Abstract: The detailed optical properties of BaFe2As2 have been determined over a wide frequency range above and below the structural and magnetic transition at T_N = 138 K. A prominent in-plane infrared-active mode is observed at 253 cm^{-1} (31.4 meV) at 295 K. The frequency of this vibration shifts discontinuously at T_N; for T < T_N the frequency of this mode displays almost no temperature dependence, yet it nearly doubles in intensity. This anomalous behavior appears to be a consequence of orbital ordering in the Fe-As layers.
Electronic correlations and unusual superconducting response in the optical properties of the iron-chalcogenide FeTe0.55Se0.45
C. C. Homes,A. Akrap,J. S. Wen,Z. J. Xu,Z. W. Lin,Q. Li,G. D. Gu
Physics , 2010, DOI: 10.1103/PhysRevB.81.180508
Abstract: The in-plane complex optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45 have been determined above and below the critical temperature Tc = 14 K. At room temperature the conductivity is described by a weakly-interacting Fermi liquid; however, below 100 K the scattering rate develops a frequency dependence in the terahertz region, signaling the increasingly correlated nature of this material. We estimate the dc conductivity just above Tc to be sigma_dc ~ 3500 Ohm-1cm-1 and the superfluid density rho_s0 ~ 9 x 10^6 cm-2, which places this material close to the scaling line rho_s0/8 ~ 8.1 sigma_dc Tc for a BCS dirty-limit superconductor. Below Tc the optical conductivity reveals two gap features at Delta_1,2 ~ 2.5 and ~ 5.1 meV.
Separation of orbital contributions to the optical conductivity of BaVS$_3$
I. Kezsmarki,G. Mihaly,R. Gaal,N. Barisic,A. Akrap,H. Berger,L. Forro,C. C. Homes,L. Mihaly
Physics , 2006, DOI: 10.1103/PhysRevLett.96.186402
Abstract: The correlation-driven metal-insulator transition (MIT) of BaVS$_3$ was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: The quasi 1D metallic transport of $A_{1g}$ electrons is superimposed on the isotropic hopping conduction of localized $E_g$ electrons. The "bad-metal" character and the weak anisotropy are the consequences of the large effective mass $m_{eff}\approx7m_e$ and scattering rate $\Gamma\geq160$ meV of the quasi-particles in the $A_{1g}$ band. There is a pseudo-gap above $T_{MI}=69$ K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with $\Delta_{ch}\approx42$ meV in the ground state. The MIT is described in terms of a weakly coupled two-band model.
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