arXiv subjects

Eun-Ah Kim

Publications and source records attributed to Eun-Ah Kim.

122 records · Page 7Linked to original sources

arXiv2008

Theory of the nodal nematic quantum phase transition in superconductors

We study the character of an Ising nematic quantum phase transition (QPT) deep inside a d-wave superconducting state with nodal quasiparticles in a two-dimensional tetragonal crystal. We find that, within a 1/N expansion, the transition is continuous. To leading order in 1/N, quantum fluctuations enhance the dispersion anisotropy of the nodal excitations, and cause strong scattering which critically broadens the quasiparticle (qp) peaks in the spectral function, except in a narrow wedge in momentum space near the Fermi surface where the qp's remain sharp. We also consider the possible existence of a nematic glass phase in the presence of weak disorder. Some possible implications for cuprate physics are also discussed.

cond-mat.supr-con↗

arXiv2008

Non-Abelian statistics in the interference noise of the Moore-Read quantum Hall state

We propose noise oscillation measurements in a double point contact, accessible with current technology, to seek for a signature of the non-abelian nature of the ν=5/2 quantum Hall state. Calculating the voltage and temperature dependence of the current and noise oscillations, we predict the non-abelian nature to materialize through a multiplicity of the possible outcomes: two qualitatively different frequency dependences of the nonzero interference noise. Comparison between our predictions for the Moore-Read state with experiments on ν=5/2 will serve as a much needed test for the nature of the ν=5/2 quantum Hall state.

cond-mat.mes-hall↗

arXiv2007

Graphene as an electronic membrane

Experiments are finally revealing intricate facts about graphene which go beyond the ideal picture of relativistic Dirac fermions in pristine two dimensional (2D) space, two years after its first isolation. While observations of rippling added another dimension to the richness of the physics of graphene, scanning single electron transistor images displayed prevalent charge inhomogeneity. The importance of understanding these non-ideal aspects cannot be overstated both from the fundamental research interest since graphene is a unique arena for their interplay, and from the device applications interest since the quality control is a key to applications. We investigate the membrane aspect of graphene and its impact on the electronic properties. We show that curvature generates spatially varying electrochemical potential. Further we show that the charge inhomogeneity in turn stabilizes ripple formation.

cond-mat.other↗

arXiv2007

Stability of half-quantum vorticies in p_x+ip_y superconductors

We consider the stability conditions for half-quantum vortices in quasi two dimensional $p_x+ip_y$ superconductor (such as ${\rm Sr_2 RuO_4}$ is believed to be) through energetics calculations. Although the predicted exotic nature of these excitations have recently attracted much attention but they have not been observed yet. We pay particular attention to the fact that an isolated half quantum vortex has a divergent energy cost in the bulk due to the associated {\it unscreened} spin current, which requires two half-quantum vortices with opposite spin winding to pair. We show that the stability of such a pair in bulk is enhanced when the ratio of spin superfluid density to superfluid density $ρ_{\rm sp}/ρ_{\rm s}$ is small. As a step towards stabilizing and observing these exotic excitations, we propose using various mesoscopic geometries. We find that certain mesoscopic geometries can selectively allow only single half-quantum vortices to enter the sample for small $ρ_{\rm sp}/ρ_{\rm s}$.

cond-mat.supr-con↗

arXiv2007

Dynamical layer decoupling in a stripe-ordered, high T_c superconductor

In the stripe-ordered state of a strongly-correlated two-dimensional electronic system, under a set of special circumstances, the superconducting condensate, like the magnetic order, can occur at a non-zero wave-vector corresponding to a spatial period double that of the charge order. In this case, the Josephson coupling between near neighbor planes, especially in a crystal with the special structure of La_{2-x}Ba_xCuO_4, vanishes identically. We propose that this is the underlying cause of the dynamical decoupling of the layers recently observed in transport measurements at x=1/8.

cond-mat.str-el↗

arXiv2006

Aharanov-Bohm interference and fractional statistics in a quantum Hall interferometer

We compute the temperature, voltage, and magnetic field dependences of the resistance oscillations of a model interferometer designed to measure the fractional statistics of the quasiparticles in the fractional quantum Hall (FQH) effect. The geometry is the same as that used in recent experiments reported in Refs. \cite{camino-prl,camino-T,zhou}. With appropriate assumptions concerning the relative areas of the inner and outer rings of the interferometer, we find theoretical results, including the existence of periodic Aharonov-Bohm oscillations with periods {\it larger} than the fundamental quantum of flux, which are in remarkably good agreement with experiment. It is then possible to make additional experimental predictions with no adjustable parameters which, if verified, would confirm the proposed interpretation of the experiment as a measurement of fractional statistics.

cond-mat.mes-hall↗

arXiv2006

Theory of stripes in quasi two dimensional rare-earth tritellurides

Even though the rare-earth tritellurides are tetragonal materials with a quasi two dimensional (2D) band structure, they have a "hidden" 1D character. The resultant near-perfect nesting of the Fermi surface leads to the formation of a charge density wave (CDW) state. We show that for this band structure, there are two possible ordered phases: A bidirectional "checkerboard" state would occur if the CDW transition temperature were sufficiently low, whereas a unidirectional "striped" state, consistent with what is observed in experiment, is favored when the transition temperature is higher. This result may also give some insight into why, in more strongly correlated systems, such as the cuprates and nickelates, the observed charge ordered states are generally stripes as opposed to checkerboards.

cond-mat.str-el↗

arXiv2006

Measuring fractional charge and statistics in fractional quantum Hall fluids through noise experiments

A central long standing prediction of the theory of fractional quantum Hall (FQH) states that it is a topological fluid whose elementary excitations are vortices with fractional charge and fractional statistics. Yet, the unambiguous experimental detection of this fundamental property, that the vortices have fractional statistics, has remained an open challenge. Here we propose a three-terminal ``T-junction'' as an experimental setup for the direct and independent measurement of the fractional charge and statistics of fractional quantum Hall quasiparticles via cross current noise measurements. We present a non-equilibrium calculation of the quantum noise in the T-junction setup for FQH Jain states. We show that the cross current correlation (noise) can be written in a simple form, a sum of two terms, which reflects the braiding properties of the quasiparticles: the statistics dependence captured in a factor of $\cosθ$ in one of two contributions. Through analyzing these two contributions for different parameter ranges that are experimentally relevant, we demonstrate that the noise at finite temperature reveals signatures of generalized exclusion principles, fractional exchange statistics and fractional charge. We also predict that the vortices of Laughlin states exhibit a ``bunching'' effect, while higher states in the Jain sequences exhibit an ``anti-bunching'' effect.

cond-mat.mes-hall↗

arXiv2005

Signatures of fractional statistics in noise experiments in quantum Hall fluids

The elementary excitations of fractional quantum Hall (FQH) fluids are vortices with fractional statistics. Yet, this fundamental prediction has remained an open experimental challenge. Here we show that the cross current noise in a three-terminal tunneling experiment of a two dimensional electron gas in the FQH regime can be used to detect directly the statistical angle of the excitations of these topological quantum fluids. We show that the noise also reveals signatures of exclusion statistics and of fractional charge. The vortices of Laughlin states should exhibit a ``bunching'' effect, while for higher states in the Jain sequences they should exhibit an ``anti-bunching'' effect.

cond-mat.mes-hall↗

arXiv2005

Quantum Hall line junction with impurities as a multi-slit Luttinger liquid interferometer

We report on quantum interference between a pair of counterpropagating quantum Hall edge states that are separated by a high quality tunnel barrier. Observed Aharonov-Bohm oscillations are analyzed in terms of resonant tunneling between coupled Luttinger liquids that creates bound electronic states between pairs of tunnel centers that act like interference slits. We place a lower bound in the range of 20-40 $μ$m for the phase coherence length and directly confirm the extended phase coherence of quantum Hall edge states.

cond-mat.mes-hall↗

arXiv2004

Cooper pair tunneling in junctions of singlet quantum Hall states and superconductors

We propose tunnel junctions of a Hall bar and a superconducting lead, for observing Cooper-pair tunneling into singlet fractional quantum Hall edge states. These tunnel junctions provide a natural means of extracting precise information of the spin polarization and the filling factor of the state. The low energy regime of one of the set-ups is governed by a novel quantum entangled fixed point.

cond-mat.mes-hall↗

arXiv2004

Phase Transitions in Models for Coupled Charge-Density Waves

Various phase transitions in models for coupled charge-density waves are investigated by means of the $ε$-expansion, mean-field theory, and Monte Carlo simulations. At zero temperature the effective action for the system with appropriate commensurability effects is mapped onto the three- or four-dimensional \XY model, depending on spatiotemporal fluctuations, under the corresponding symmetry-breaking fields. It is revealed that the three- and four-dimensional systems display a single transition between the clock order (with broken Z$_M$ symmetry) and disorder. The nature of the phase transition depends crucially on the commensurability factor $M$: For $M \ge 4$, in particular, the transition belongs to the same university class as the \XY model. On the other hand, in the presence of misfit causing frustration in the charge-density wave, the inter-chain coupling is observed to favor either the commensurate state or the incommensurate state depending on the initial configuration; this gives rise to hysteresis around the commensurate-incommensurate transition. Boundaries separating such phases with different symmetries are obtained in the parameter space consisting of the temperature, symmetry-breaking field, fluctuation strength, inter-chain coupling, and misfit.

cond-mat.str-el↗

arXiv2003

Double point contact in Quantum Hall Line Junctions

We show that multiple point contacts on a barrier separating two laterally coupled quantum Hall fluids induce Aharonov-Bohm (AB) oscillations in the tunneling conductance. These quantum coherence effects provide new evidence for the Luttinger liquid behavior of the edge states of quantum Hall fluids. For a two point contact, we identify coherent and incoherent regimes determined by the relative magnitude of their separation and the temperature. We analyze both regimes in the strong and weak tunneling amplitude limits as well as their temperature dependence. We find that the tunneling conductance should exhibit AB oscillations in the coherent regime, both at strong and weak tunneling amplitude with the same period but with different functional form.

cond-mat.mes-hall↗

arXiv2002

Inter edge Tunneling in Quantum Hall Line Junctions

We propose a scenario to understand the puzzling features of the recent experiment by Kang and coworkers on tunneling between laterally coupled quantum Hall liquids by modeling the system as a pair of coupled chiral Luttinger liquid with a point contact tunneling center. We show that for filling factors $ν\sim1$ the effects of the Coulomb interactions move the system deep into strong tunneling regime, by reducing the magnitude of the Luttinger parameter $K$, leading to the appearance of a zero-bias differential conductance peak of magnitude $G_t=Ke^2/h$ at zero temperature. The abrupt appearance of the zero bias peak as the filling factor is increased past a value $ ν^* \gtrsim 1$, and its gradual disappearance thereafter can be understood as a crossover controlled by the main energy scales of this system: the bias voltage $V$, the crossover scale $T_K$, and the temperature $T$. The low height of the zero bias peak $\sim 0.1e^2/h$ observed in the experiment, and its broad finite width, can be understood naturally within this picture. Also, the abrupt reappearance of the zero-bias peak for $ν\gtrsim 2$ can be explained as an effect caused by spin reversed electrons, \textit{i. e.} if the 2DEG is assumed to have a small polarization near $ν\sim2$. We also predict that as the temperature is lowered $ν^*$ should decrease, and the width of zero-bias peak should become wider. This picture also predicts the existence of similar zero bias peak in the spin tunneling conductance near for $ν\gtrsim 2$.

cond-mat.mes-hall↗