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Search Results: 1 - 10 of 452306 matches for " E. S. Choi "
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Angular dependent magnetothermopower of alpha-(ET)2KHg(SCN)4
D. Krstovska,E. Steven,E. S. Choi,J. S. Brooks
Physics , 2011, DOI: 10.1063/1.3670035
Abstract: The magnetic field and angular dependencies of the thermopower and Nernst effect of the quasi-two-dimensional organic conductor alpha-(ET)2KHg(SCN)4 are experimentally measured at temperatures below (4 K) and above (9 K) the transition temperature to fields of In addition, a theoretical model which involves a magnetic breakdown effect between the q1D and q2D bands is proposed in order to simulate the data. Analysis of the background components of the thermopower and Nernst effect imply that at low temperatures, in the CDW state, the properties of alpha-(ET)2KHg(SCN)4 are determined mostly by the orbits on the new open Fermi sheets. Quantum oscillations observed in the both thermoelectric effects, at fields above 8 T, originate only from the alpha orbit.
Thermoelectric power of MgB$_{2-x}$Be$_x$
J. S. Ahn,E. S. Choi,W. Kang,D. J. Singh,E. J. Choi
Physics , 2002, DOI: 10.1103/PhysRevB.65.214534
Abstract: We investigated thermoelectric power $S(T)$ of MgB$_{2-x}$Be$_{x}$ ($x=0$, 0.2, 0.3, 0.4, and 0.6). $S(T)$ decreases systematically with $x$, suggesting that the hole density increases. Our band calculation shows that the increase occurs in the $\sigma $-band. With the hole-doping, $T_{c}$ decreases. Implication of this phenomenon is discussed within the BCS framework. While the Mott formula explains only the linear part of $S(T)$ at low temperature, incorporation of electron-phonon interaction enables us to explain $S(T)$ over wide temperature range including the anomalous behavior at high temperature.
Magnetothemopower study of quasi two-dimensional organic conductor $α$-(BEDT-TTF)$_2$KHg(SCN)$_4$
E. S. Choi,J. S. Brooks,J. S. Qualls
Physics , 2001, DOI: 10.1103/PhysRevB.65.205119
Abstract: We have used a low-frequency magneto-thermopower (MTEP) method to probe the high magnetic field ground state behavior of $\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$ along all three principal crystallographic axes at low temperatures. The thermopower tensor coefficients ($S_{xx}, S_{yx}$ and $S_{zz}$) have been measured to 30 T, beyond the anomalous low temperature, field-induced transition at 22.5 T. We find a significant anisotropy in the MTEP signal, and also observe large quantum oscillations associated with the de Haas - van Alphen effect. The anisotropy indicates that the ground state properties are clearly driven by mechanisms that occur along specific directions for the in-plane electronic structure. Both transverse and longitudinal magnetothermopower show asymptotic behavior in field, which can be explained in terms of magnetic breakdown of compensated closed orbits.
Metallic temperature dependence of resistivity in perchlorate doped polyacetylene
Y. W. Park,E. S. Choi,D. S. Suh
Physics , 1998,
Abstract: We have measured the electrical resistivity ($\rho$) and the thermoelectric power (TEP) of the perchlorate (ClO4^-) doped stretch oriented polyacetylene (PA) film. For the highly conducting samples ($\sigma_{RT} > 41000 S/cm$), the temperature dependence of the 4-probe resistivity shows positive temperature coefficient of resistivity (TCR) from T=1.5K to 300K. For the less conducting samples, the 4-probe resistivity data show the crossover of TCR with a broad minimum peak at T=T* > 200K. For samples of $\sigma_{RT}$$>$20000 S/cm, the $\rho (1.5K)/\rho (300K) <1$, i.e., the resistivity at 1.5K is lower than the room temperature resistivity value. The temperature dependence of the TEP shows diffusive linear metallic TEP becoming temperature independent below 40K. Unlike the others who used Cu(ClO_4)_2 for the ClO_4^- doping, the initial doping material we used is anhydrous Fe(ClO_4)_3 which is crucial to obtain the positive TCR from T=1.5K to 300K.
Thermally activated energy and flux flow Hall effect of Fe1+y(Te1-xSx)z
Hechang Lei,Rongwei Hu,E. S. Choi,C. Petrovic
Physics , 2010, DOI: 10.1103/PhysRevB.82.134525
Abstract: Thermally activated flux flow (TAFF) and flux flow Hall effect (FFHE) of Fe(Te,S) single crystal in the mixed state are studied in magnetic fields up to 35 T. Thermally activated energy (TAE) is analyzed using conventional Arrhenius relation and modified TAFF theory which is closer to experimental results. The results indicate that there is a crossover from single-vortex pinning region to collective creep pinning region with increasing magnetic field. The temperature dependence of TAE is different for H//ab and H//c. On the other hand, the analysis of FFHE in the mixed state indicates that there is no Hall sign reversal. We also observe scaling behavior |Rhoxy(H)| = A*Rhoxx(H)^beta.
Low-frequency method for magnetothermopower and Nernst effect measurements on single crystal samples at low temperatures and high magnetic fields
E. S. Choi,J. S. Brooks,J. S. Qualls,Y. S. Song
Physics , 2000, DOI: 10.1063/1.1353192
Abstract: We describe an AC method for the measurement of the longitudinal (Sxx) and transverse (Sxy, i.e. Nernst) thermopower of mm-size single crystal samples at low temperatures (T<1 K) and high magnetic fields (B>30 T). A low-frequency (33 mHz) heating method is used to increase the resolution, and to determine the temperature gradient reliably in high magnetic fields. Samples are mounted between two thermal blocks which are heated by a sinusoidal frequency f0 with a p/2 phase difference. The phase difference between two heater currents gives a temperature gradient at 2f0. The corresponding thermopower and Nernst effect signals are extracted by using a digital signal processing method due. An important component of the method involves a superconducting link, YBa2Cu3O7+d (YBCO), which is mounted in parallel with sample to remove the background magnetothermopower of the lead wires. The method is demonstrated for the quasi two-dimensional organic conductor a-(BEDT-TTF)2KHg(SCN)4, which exhibits a complex, magnetic field dependent ground state above 22.5 T at low temperatures.
Transverse and in-plane modification of superconductivity and electronic structure in the quasi-two dimensional organic conductor $κ $--(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$ by uniaxial stress
E. S. Choi,J. S. Brooks,S. Y. Han,L. Balicas,J. S. Qualls
Physics , 2000, DOI: 10.1080/13642810110033827
Abstract: We have employed uniaxial stress along the principal axes of the quasi-two dimensional organic superconductor $\kappa $--(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$. The lattice anisotropy is thereby altered, with corresponding changes in the intermolecular transfer energies. The effect of uniaxial stress on the superconducting transition temperature $T_{c}$ and critical field $B_{c2}$ is found to be anisotropic.There is an indication of an increase in $T_{c}$ and $B_{c2}$ for in-plane stress, but both parameters decrease rapidly for transverse (inter-plane) stress. Magnetotransport studies reveal stress-induced changes in the Fermi surface through the observation of the Shubnikov de Haas oscillations. The stress dependence of a resistive anomaly in the magnetoresistance, which is associated with the critical field $B_{c2}$, is also investigated. We discuss the experimental findings in the context of recent phenomenological and theoretical treatments of quasi-two dimensional systems where the anisotropic triangular lattice Hubbard model has been used to treat two-dimensional superconductors.
Magnetic, electronic and Shubnikov-de Haas investigation of the dense Kondo system CeAgSb2
E. Jobiliong,J. S. Brooks,E. S. Choi,Han-Oh Lee,Z. Fisk
Physics , 2005,
Abstract: Of the dense Kondo materials in the class CeTSb2 (where T = Au, Ag, Ni, Cu, or Pd), CeAgSb2 is special due to its complex magnetic ground state, which exhibits both ferro- and anti-ferromagnetic character below an ordering temperature TO ~ 9.8 K. To further elucidate a description this magnetic ground state, we have carried out a systematic study of single crystalline CeAgSb2 by magnetic, electrical magneto-transport, and Shubnikov-de Haas (SdH) studies over a broad range of temperature and magnetic field. We have constructed the magnetic phase diagram based solely on magnetoresistance data. Here, depending on the orientation of the magnetic field H, either ferromagnetic or antiferromagnetic ordering occurs below TO. The resistivity of this compound below TO does not follow a simple Fermi liquid behavior, but requires an additional contribution from conduction electron scattering from boson excitations with an energy gap, D. At zero field the temperature dependent resistivity below TO is most consistent with antiferromagnetic order, based on the transport theory which includes magnon scattering. Crystal field effect theory applied to the susceptibility data yields splitting energies from the ground state to the first and second excited states of 53 K and 137 K, respectively. Although there is some uncertainty in the Kondo temperature determination, we estimate TK ~ 23 K from our analysis. In the Fermi surface studies, the measurements show very small Fermi surface sections, not predicted by band structure calculations, and the SdH amplitudes are very sensitive to field direction. Only by considering lens orbits between the main Fermi surface cylinders can the SdH results be reconciled with the Fermi surface topology predicted from band structure.
High field alloy, thermoelectric, and mm wave studies of the field induced superconducting State in l-(BETS)2FexGa1-xCl4
Brooks J.S.,Uji S.,Choi E.S.,Rutel I.B.
Brazilian Journal of Physics , 2003,
Abstract: Alloy studies in the pi - d organic conductor lambda-(BETS)2Fe xGa1-xCl4 have given new insight into the nature of field induced superconductivity (FISC), since the mechanism of the FISC involves cancellation of the pi - d exchange field by the external field. Alloying on the Fe xGa1-x site allows tuning of the exchange field, thereby influencing the FISC phase boundary. A brief review of the low temperature phases are given, and new high magnetic field thermoelectric and mm wave results that probe the low temperature ground state are presented.
Structural and superconducting properties of MgB$_{2-x}$Be$_x$
J. S. Ahn,Y. -J. Kim,M. -S. Kim,S. -I. Lee,E. J. Choi
Physics , 2002, DOI: 10.1103/PhysRevB.65.172503
Abstract: We prepared MgB$_{2-x}$Be$_{x}$ ($x=0$, 0.2, 0.3, 0.4, and 0.6) samples where B is substituted with Be. MgB$_{2}$ structure is maintained up to $x=0.6$. In-plane and inter-plane lattice constants were found to decrease and increase, respectively. Superconducting transition temperature $T_{c}$ decreases with $x$. We found that the $T_{c}$ decrease is correlated with in-plane contraction but is insensitive to carrier doping, which is consistent with other substitution studies such as Mg$_{1-x}$Al$_{x}$B$_{2}$ and MgB$_{2-x}$C$_{x}$. Implication of this work is discussed in terms of the 2D nature of $\sigma $-band.
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