arXiv subjects

Mu-Yong Choi

Publications and source records attributed to Mu-Yong Choi.

9 recordsLinked to original sources

arXiv2016

Hund's metallicity and orbital-selective Mott localization of CrO$_{2}$ in the paramagnetic state

We present electronic structure calculations of a CrO$_{2}$ compound in the paramagnetic state within the computational scheme of density-functional theory combined with dynamical mean-field theory. We find that CrO$_{2}$ in the paramagnetic state is a strongly-correlated Hund's metal with mixed-valent Cr ions. At high temperatures, CrO$_{2}$ shows an orbital-selective Mott phase, in which the Cr $d_{xy}$ orbital is Mott-localized while the other Cr $t_{2g}$ $d$ orbitals remain itinerant. When the temperature is lowered, the orbital-selective Mott state is found to evolve into a Kondo-like heavy-fermion state with a small pseudo gap at the Fermi level if the system remains in the paramagnetic state.

cond-mat.str-el↗

arXiv2008

Superconducting phase transitions in frustrated Josephson-junction arrays on a dice lattice

Transport measurements are carried out on dice Josephson-junction arrays with the frustration index $f=1/3$ and 1/2 which possess, within the limit of the $XY$ model, an accidental degeneracy of the ground states as a consequence of the formation of zero-energy domain walls. The measurements demonstrate that both the systems undergo a phase transition to a superconducting vortex-ordered state at considerably high temperatures. The experimental findings are in apparent contradiction with the theoretical expectation that frustration effects in the $f=1/3$ system are particularly strong enough to suppress a vortex-ordering transition down to near zero temperature. The data for $f=1/2$ are more consistent with theoretical evaluations. The agreement between the experiments and the Monte Carlo simulations of a $XY$ model for $f=1/3$ suggests that the order-from-disorder mechanism for the removal of an accidental degeneracy may still be effective in the $f=1/3$ system. The transport data also reveal that the dice arrays with zero-energy domain walls experience a much slower critical relaxation than other frustrated arrays only with finite-energy walls.

cond-mat.supr-con↗

arXiv2006

Phase Transition and Critical Dynamics in Diluted Josephson Junction Arrays

Measurements of the $IV$ characteristics of site-diluted Josephson-junction arrays have revealed intriguing effects of percolative disorder on the phase transition and the vortex dynamics in a two-dimensional XY system. Different from other types of phase transitions, the Kosterlitz-Thouless (KT) transition was eliminated with the introduction of percolative disorder far below the percolation threshold. Even after the KT order had been removed, the system remained superconducting at low temperatures by establishing a different type of order. Near the percolation threshold, evidence was found that, as a consequence of the underlying fractal structure, the critical dynamics of the phase degrees of freedom persisted down to zero temperature.

cond-mat.dis-nn↗

arXiv2005

Experimental Study of Positionally Disordered Josephson Junctions Arrays

We experimentally studied the effect of positional disorder on a Josephson junction array with $f = n$, ${1/2} + n$, or ${2/5} + n$ flux quanta per unit cell for integral $n$. This system provides an experimental realization of a two-dimensional XY model with random phase shifts. Contrary to many earlier numerical and analytical investigations, our results suggest that low-temperature superconductivity is never destroyed by positional disorder. As the disorder strength increased, the Kosterltz-Thouless (KT) type order in the $f = 0$ and 1/2 systems changed to a non-KT type order with a long-range phase coherence, which persisted even in the maximal disorder limit. A possible finite-temperature glass transition is discussed.

cond-mat.supr-con↗

arXiv2005

Superconducting transition of a two-dimensional Josephson junction array in weak magnetic fields

The superconducting transition of a two-dimensional (2D) Josephson junction array exposed to weak magnetic fields has been studied experimentally. Resistance measurements reveal a superconducting-resistive phase boundary in serious disagreement with the theoretical and numerical expectations. Critical scaling analyses of the $IV$ characteristics indicate contrary to the expectations that the superconducting-to-resistive transition in weak magnetic fields is associated with a melting transition of magnetic-field-induced vortices directly from a pinned-solid phase to a liquid phase. The expected depinning transition of vortices from a pinned-solid phase to an intermediate floating-solid phase was not observed. We discuss effects of the disorder-induced random pinning potential on phase transitions of vortices in a 2D Josephson junction array.

cond-mat.supr-con↗

arXiv2004

Frustrated two-dimensional Josephson junction array near incommensurability

To study the properties of frustrated two-dimensional Josephson junction arrays near incommensurability, we examine the current-voltage characteristics of a square proximity-coupled Josephson junction array at a sequence of frustrations f=3/8, 8/21, 0.382 $(\approx (3-\sqrt{5})/2)$, 2/5, and 5/12. Detailed scaling analyses of the current-voltage characteristics reveal approximately universal scaling behaviors for f=3/8, 8/21, 0.382, and 2/5. The approximately universal scaling behaviors and high superconducting transition temperatures indicate that both the nature of the superconducting transition and the vortex configuration near the transition at the high-order rational frustrations f=3/8, 8/21, and 0.382 are similar to those at the nearby simple frustration f=2/5. This finding suggests that the behaviors of Josephson junction arrays in the wide range of frustrations might be understood from those of a few simple rational frustrations.

cond-mat.supr-con↗

arXiv2002

Critical Behavior of Frustrated Josephson Junction Arrays with Bond Disorder

The scaling behavior of the current-voltage ($IV$) characteristics of a two-dimensional proximity-coupled Josephson junction array (JJA) with quenched bond disorder was investigated for frustrations $f=1/5$, 1/3, 2/5, and 1/2. For all these frustrations including 1/5 and 2/5 where a strongly first-order phase transition is expected in the absence of disorder, the $IV$ characteristics exhibited a good scaling behavior. The critical exponent $ν$ indicates that bond disorder may drive the phase transitions of frustrated JJA's to be continuous but not into the Ising universality class, contrary to what was observed in Monte Carlo simulations. The dynamic critical exponent $z$ for JJA's was found to be only 0.60 - 0.77.

cond-mat.supr-con↗

arXiv2000

Unusual T_c variation with hole concentration in Bi_2Sr_{2-x}La_xCuO_{6+δ}

We have investigated the $T_c$ variation with the hole concentration $p$ in the La-doped Bi 2201 system, Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$. It is found that the Bi 2201 system does not follow the systematics in $T_c$ and $p$ observed in other high-$T_c$ cuprate superconductors (HTSC's). The $T_c$ vs $p$ characteristics are quite similar to what observed in Zn-doped HTSC's. An exceptionally large residual resistivity component in the inplane resistivity indicates that strong potential scatterers of charge carriers reside in CuO$_2$ planes and are responsible for the unusual $T_c$ variation with $p$, as in the Zn-doped systems. However, contrary to the Zn-doped HTSC's, the strong scatter in the Bi 2201 system is possibly a vacancy in the Cu site.

cond-mat.supr-con↗

arXiv1998

Thermopower of high-T_c cuprates

We have studied the thermopower of La_2CuO_{4+z} and Nd_2CuO_{4-y} which undergo an antiferromagnetic transition near room temperature and Bi_2Sr_{2-x}La_xCuO_{6+z} for which a broad spectrum of doping is possible. The thermopower of La_2CuO_{4+z} and Nd_2CuO_{4-y} is seen to exhibit an anomaly at the Neel temperature, whereas the resistivity is not. The extrapolated zero-temperature intercept of the thermopower, which is known to be positive for hole-doped cuprate superconductors, is found to become negative for Bi_2Sr_{2-x}La_xCuO_{6+z} above some critical doping-level. The data strongly suggest that the thermopower of high-T_c cuprates contains a large amount of extra contribution in addition to the usual diffusion thermopower. We discuss origins of the extra contribution in the thermopower.

cond-mat.supr-con↗