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Yong Baek Kim

Publications and source records attributed to Yong Baek Kim.

252 records · Page 14Linked to original sources
arXiv1999

Fermions in an anisotropic random magnetic field

We study the localization of fermions in an anisotropic random magnetic field in two dimensions. It is assumed that the randomness in a particular direction is stronger than those in the other directions. We consider a network model of zero field contours, where there are two types of randomness - the random tunneling matrix element at the saddle points and unidirectional random variation of the number of fermionic states following zero field contours. After averaging over the random complex tunneling amplitude, the problem is mapped to an SU(2N) random exchange quantum spin chain in the $N \to 0$ limit. We suggest that the fermionic state becomes critical in an anisotropic fashion.

cond-mat.mes-hall↗
arXiv1999

Berry Phase Theory of Anomalous Hall Effect: Application to Colossal Magnetoresistance Manganites

We show that the Anomalous Hall Effect (AHE) observed in Colossal Magnetoresistance Manganites is a manifestation of Berry phase effects caused by carrier hopping in a non-trivial spin background. We determine the magnitude and temperature dependence of the Berry phase contribution to the AHE, finding that it increases rapidly in magnitude as the temperature is raised from zero through the magnetic transition temperature T_c, peaks at a temperature $T_{max} > T_c$ and decays as a power of T, in agreement with experimental data. We suggest that our theory may be relevant to the anomalous hall effect in conventional ferromagnets as well.

cond-mat.str-el↗
arXiv1999

Coulomb drag as a signature of the paired quantum Hall state

Motivated by the recent Coulomb drag experiment of M. P. Lilly et. al, we study the Coulomb drag in a two-layer system with Landau level filling factor $ν=1/2$. We find that the drag conductivity in the incompressible paired quantum Hall state at zero temperature can be finite. The drag conductivity is also greatly enhanced above $T_c$, at which the transition between the weakly coupled compressible liquids and the paired quantum Hall liquid takes place. We discuss the implications of our results for the recent experiment.

cond-mat.mes-hall↗
arXiv1998

Incommensurate Charge and Spin Fluctuations in d-wave Superconductors

We show analytic results for the irreducible charge and spin susceptibilities, $χ_0 (ω, {\bf Q})$, where ${\bf Q}$ is the momentum transfer between the nodes in d-wave superconductors. Using the BCS theory and a circular Fermi surface, we find that the singular behavior of the irreducible charge susceptibility leads to the dynamic incommensurate charge collective modes. The peaks in the charge structure factor occur at a set of wave vectors which form an ellipse around ${\bf Q}_π=(π,π)$ and ${\bf Q}_0=(0,0)$ in momentum space with momentum dependent spectral weight. It is also found that, due to the non-singular irreducible spin susceptibility, an extremely strong interaction via random phase approximation is required to support the magnetic peaks near ${\bf Q}_π$. Under certain conditions, the peaks in the magnetic structure factor occur near ${\bf Q}=(π,π(1 \pm δ))$ and $(π(1 \pm δ),π)$.

cond-mat.supr-con↗
arXiv1998

Anomalous Hall Effect in Double Exchange Magnets

We investigate the possible origin of anomalous Hall effect in the CMR (colossal magnetoresistance) materials - the doped rare earth manganites - observed recently by Matl et al. It is demonstrated that the spin-orbit interaction in the double exchange model couples magnetization to the Berry phase associated with three dimensional spin textures and induces a non-zero average topological flux which in turn generates an anomalous contribution to transverse resistivity. The same effect, but involving the orbital Berry phase, occurs in the model with orbital degeneracy and Coulomb repulsion.

cond-mat.str-el↗
arXiv1997

Conductance fluctuations at the fractional quantum Hall plateau transitions

We obtain a ``mean field'' scaling flow of the longitudinal and the Hall conductivities in the fractional quantum Hall regime. Using the composite fermion picture and assuming that the composite fermions follow the Khmelnitskii-Pruisken scaling flow for the integer quantum Hall effect, the unstable fixed points which govern the transitions between different fractional quantum Hall states are identified. Distributions of the critical longitudinal and Hall conductivities at the unstable fixed points are obtained and implications of the results for the experiments on mesoscopic quantum Hall systems are discussed.

cond-mat.mes-hall↗
arXiv1996

Gauge drag between half-filled Landau levels

The transresistance (i.e. the voltage induced in one layer by a current in another) between composite fermions in double-layers of half-filled Landau levels is shown to be dominated by scattering due to singular gauge field fluctuations arising from the antisymmetric combination of density fluctuations in two layers. The drag rate is found to be proportional to $T^{4/3}$ for $T$ less than an energy scale $Ω_{cr}$ which depends on the spacing, $d$, between the layers. An observation of a $T^{4/3}$ temperature dependence in the transresistance would be a strong evidence for the existence of the gauge field fluctuations.

cond-mat.str-el↗
arXiv1996

Overscreened Single Channel Kondo Problem

We consider the single channel Kondo problem with the Kondo coupling between a spin $S$ impurity and conduction electrons with spin $j$. These problems arise as multicritical points in the parameter spaces of two- and higher-level tunneling systems, and some impurity models of heavy fermion compounds. In contrast to the previous Bethe-anstaz conjectures, it turns out that the dynamics of the spin sector is the same as that of a spin $S$ impurity coupled to $k(j)$ channels of spin $1/2$ electrons with $k(j) = 2j(j+1)(2j+1)/3$. As a result, for $2S < k(j)$, the system shows non-Fermi liquid behavior with the same exponents for the thermodynamic quantities as those of $k(j)$ channel Kondo problem. However, both the finite-size spectrum and the operator content are different due to the presence of the other sectors and can be obtained by conformal field theory techniques.

cond-mat↗
arXiv1996

Specific heat and validity of quasiparticle approximation in the half-filled Landau level

We calculate the specific heat of composite fermion system in the half-filled Landau level. Two different methods are used to examine validity of the quasiparticle approximation when the two-body interaction is given by $V(q) = V_0 / q^{2-η}$ ($1 \le η\le 2$). The singular part of the specific heat is calculated from the free energy of the gauge field, which is compared with the specific heat calculated from the quasiparticle approximation via the singular self-energy correction due to the gauge field fluctuations. It turns out that two results are in general different and they coincide only for the case of the Coulomb interaction ($η= 1$). This result supports the fact that the quasiparticle approximation is valid only for the case of the Coulomb interaction. It is emphasized that this result is obtained by looking at a gauge-invariant quantity -- the specific heat.

cond-mat↗
arXiv1996

Edge states of integral quantum Hall states versus edge states of antiferromagnetic quantum spin chains

Using the network model representation, it is shown that the edge states of finite-size integral quantum Hall liquid can be regarded as the edge states of an SU$(2N)$ open antiferromagnetic quantum spin chain in the $N \rightarrow 0$ limit. The structures of edge states in both cases of integer quantum Hall liquid and an SU$(2N)$ antiferromagnetic quantum spin chain are compared and the relations between them are pointed out. This correspondence is used to give qualitative arguments in favor of the recent results on two-dimensional electron systems coupled in layers with a large perpendicular magnetic field. In particular, it is shown that the absence of the localization in the two-dimensional chiral surface state of the integral quantum Hall liquid in the case of the finite-size coupled system can be explained by the absence of the gap in the excitation spectrum of an SU$(2N)$ ferromagnetic quantum spin chain in the $N \rightarrow 0$ limit.

cond-mat↗
arXiv1995

On a Network Model of Localization in a Random Magnetic Field

We consider a network model of snake states to study the localization problem of non-interacting fermions in a random magnetic field with zero average. After averaging over the randomness, the network of snake states is mapped onto $M$ coupled SU$(2N)$ spin chains in the $N \rightarrow 0$ limit. The number of snake states near the zero-field contour, $M$, is an even integer. In the large conductance limit $g = M {e^2 \over 2 π\hbar}$ ($M \gg 2$), it turns out that this system is equivalent to a particular representation of the ${\rm U}(2N) / {\rm U}(N) \times {\rm U}(N)$ sigma model ($N \rightarrow 0$) {\it without} a topological term. The beta function $β(1/M)$ of this sigma model in the $1/M$ expansion is consistent with the previously known $β(g)$ of the unitary ensemble. These results and further plausible arguments support the conclusion that all the states are localized.

cond-mat↗
arXiv1995

Quantum Boltzmann equation of composite fermions interacting with a gauge field

We derive the quantum Boltzmann equation (QBE) of composite fermions at/near the $ν= 1/2$ state using the non-equilibrium Green's function technique. The lowest order perturbative correction to the self-energy due to the strong gauge field fluctuations suggests that there is no well defined Landau-quasi-particle. Therefore, we cannot assume the existence of the Landau-quasi-particles {\it a priori} in the derivation of the QBE. Using an alternative formulation, we derive the QBE for the generalized Fermi surface displacement which corresponds to the local variation of the chemical potential in momentum space. {}From this QBE, one can understand in a unified fashion the Fermi-liquid behaviors of the density-density and the current-current correlation functions at $ν= 1/2$ (in the long wave length and the low frequency limits) and the singular behavior of the energy gap obtained from the finite temperature activation behavior of the compressibility near $ν= 1/2$. Implications of these results to the recent experiments are also discussed.

cond-mat↗
arXiv1994

Gauge-invariant response functions of fermions coupled to a gauge field

We study a model of fermions interacting with a gauge field and calculate gauge-invariant two-particle Green's functions or response functions. The leading singular contributions from the self-energy correction are found to be cancelled by those from the vertex correction for small $q$ and $Ω$. As a result, the remaining contributions are not singular enough to change the leading order results of the random phase approximation. It is also shown that the gauge field propagator is not renormalized up to two-loop order. We examine the resulting gauge-invariant two-particle Green's functions for small $q$ and $Ω$, but for all ratios of $Ω/ v_F q$ and we conclude that they can be described by Fermi liquid forms without a diverging effective mass.

cond-mat↗
arXiv1994

Influence of gauge-field fluctuations on composite fermions near the half-filled state

Taking into account the transverse gauge field fluctuations, which interact with composite fermions, we examine the finite temperature compressibility of the fermions as a function of an effective magnetic field $ΔB = B - 2 n_e hc/e$ ($n_e$ is the density of electrons) near the half-filled state. It is shown that, after including the lowest order gauge field correction, the compressibility goes as ${\partial n \over \partial μ} \propto e^{- Δω_c / 2 T} \left ( 1 + {A (η) \over η- 1} {(Δω_c)^{2 \over 1 + η} \over T} \right )$ for $T \ll Δω_c$, where $Δω_c = {e ΔB \over mc}$. Here we assume that the interaction between the fermions is given by $v ({\bf q}) = V_0 / q^{2 - η} \ (1 \le η\le 2)$, where $A (η)$ is a $η$ dependent constant. This result can be interpreted as a divergent correction to the activation energy gap and is consistent with the divergent renormalization of the effective mass of the composite fermions.

cond-mat↗
arXiv1994

Test Experiment for Time-Reversal Symmetry Breaking Superconductivity

A new experiment is proposed to probe the time-reversal symmetry of a superconductor. It is shown that a time-reversal symmetry breaking superconductor can be identified by the observation of a fractional flux in connection with a Josephson junction in a special geometry.

cond-mat↗
arXiv1994

Instantons and the spectral function of electrons in the half-filled Landau level

We calculate the instanton-anti-instanton action $S_{M {\bar M}} (τ)$ in the gauge theory of the half-filled Landau level. It is found that $S_{M {\bar M}} (τ) = (3 - η) \left [ Ω_0 (η) \ τ\right ]^{1 / (3 - η)}$ for a class of interactions $v ({\bf q}) = V_0 / q^η \ ( 0 \leq η< 2 )$ between electrons. This means that the instanton-anti-instanton pairs are confining so that a well defined `charged' composite fermion can exist. It is also shown that $S_{M {\bar M}} (τ)$ can be used to calculate the spectral function of electrons from the microscopic theory within a semiclassical approximation. The resulting spectral function varies as $e^{ - \left [ Ω_0 (η) / ω\right ]^{1 / ( 2 - η) } }$ at low energies.

cond-mat↗