arXiv subjects

Jong E. Han

Publications and source records attributed to Jong E. Han.

31 records · Page 2Linked to original sources

arXiv2019

Tunable magnetic textures in spin valves: From spintronics to Majorana bound states

Spin-valve structures in which a change of magnetic configuration is responsible for magnetoresistance have enabled impressive advances in spintronics, focusing on magnetically storing and sensing information. However, this mature technology also offers versatile control of the underlying fringing fields and entirely different applications by realizing topologically-nontrivial states. Together with proximity-induced superconductivity in a two-dimensional electron gas, these fringing fields realized in commercially-available spin valves could control Majorana bound states (MBS). Detailed support for the existence and control of MBS is obtained by combining accurate micromagnetic simulation of fringing fields used as an input in Bogoliubov de Gennes equation to calculate low-energy spectrum, wavefunction localization, and local charge neutrality. A generalized condition for quantum phase transition in these structures provides valuable guidance for the MBS evolution and implementing reconfigurable effective topological wires.

cond-mat.supr-con↗

arXiv2018

Nonequilibrium Excitations and Transport of Dirac Electrons in Electric-Field-Driven Graphene

We investigate nonequilibrium excitations and charge transport in charge-neutral graphene driven with DC electric field by using the nonequilibrium Green's function technique. Due to the vanishing Fermi surface, electrons are subject to non-trivial nonequilibrium excitations such as highly anisotropic momentum distribution of electron-hole pairs, an analog of the Schwinger effect. We show that the electron-hole excitations, initiated by the Landau-Zener tunneling with a superlinear IV relation $I \propto E^{3/2}$, reaches a steady-state dominated by the dissipation due to optical phonons, resulting in a marginally sublinear IV with $I \propto E$, in agreement with recent experiments. The linear IV starts to show the sign of current saturation as the graphene is doped away from the Dirac point, and recovers the semi-classical relation for the saturated velocity. We give a detailed discussion on the nonequilibrium charge creation and the relation between the electron-phonon scattering rate and the electric field in the steady-state limit. We explain how the apparent Ohmic IV is recovered near the Dirac point. We propose a mechanism where the peculiar nonequilibrium electron-hole creation can be utilized in a novel infra-red device.

cond-mat.mes-hall↗

arXiv2018

Nonequilibrium Mean-Field Theory of Resistive Phase Transitions

We investigate the quantum mechanical origin of resistive phase transitions in solids driven by a constant electric field in the vicinity of a metal-insulator transition. We perform a nonequilibrium mean-field analysis of a driven-dissipative anti-ferromagnet, which we solve analytically for the most part. We find that the insulator-to-metal transition (IMT) and the metal-to-insulator transition (MIT) proceed by two distinct electronic mechanisms: Landau-Zener processes, and the destabilization of metallic state by Joule heating, respectively. However, we show that both regimes can be unified in a common effective thermal description, where the effective temperature $T_{\rm eff}$ depends on the state of the system. This explains recent experimental measurements in which the hot-electron temperature at the IMT was found to match the equilibrium transition temperature. Our analytic approach enables us to formulate testable predictions on the non-analytic behavior of $I$-$V$ relation near the insulator-to-metal transition. Building on these successes, we propose an effective Ginzburg-Landau theory which paves the way to incorporating spatial fluctuations, and to bringing the theory closer to a realistic description of the resistive switchings in correlated materials.

cond-mat.str-el↗

arXiv2017

Microscopic Theory of Resistive Switching in Ordered Insulators: Electronic vs. Thermal Mechanism

We investigate the dramatic switch of resistance in ordered correlated insulators, when driven out of equilibrium by a strong voltage bias. Microscopic calculations on a driven-dissipative lattice of interacting electrons explain the main experimental features of resistive switching (RS), such as the hysteretic $I$-$V$ curves and the formation of hot conductive filaments. The energy-resolved electron distribution at the RS reveals the underlying nonequilibrium electronic mechanism, namely Landau-Zener tunneling, and also justifies a thermal description where the hot-electron temperature, estimated from the first moment of the distribution, matches the equilibrium phase transition temperature. We discuss the tangled relationship between filament growth and negative differential resistance, and the influence of crystallographic structure and disorder in the RS.

cond-mat.str-el↗

arXiv2016

Tunneling Planar Hall Effect in Topological Insulators: Spin-Valves and Amplifiers

We investigate tunneling across a single ferromagnetic barrier on the surface of a three-dimensional topological insulator. In the presence of a magnetization component along the bias direction, a tunneling planar Hall conductance (TPHC), transverse to the applied bias, develops. Electrostatic control of the barrier enables a giant Hall angle, with the TPHC exceeding the longitudinal tunneling conductance. By changing the in-plane magnetization direction it is possible to change the sign of both the longitudinal and transverse differential conductance without opening a gap in the topological surface state. The transport in a topological insulator/ferromagnet junction can thus be drastically altered from a simple spin-valve to an amplifier.

cond-mat.mes-hall↗

arXiv2013

Nodal "ground states" and orbital textures in semiconductor quantum dots

Conventional understanding implies that the ground state of a nonmagnetic quantum mechanical system should be nodeless. While this notion also provides a valuable guidance in understanding the ordering of energy levels in semiconductor nanostructures, there are reports that $\textit{nodal}$ ground states for holes are possible. However, the existence of such nodal states has been debated and even viewed merely as an artifact of a $\boldsymbol{k{\cdot}p}$ model. Using complementary approaches of both $\boldsymbol{k{\cdot}p}$ and tight-binding models, further supported by an effective Hamiltonian for a continuum model, we reveal that the nodal ground states in quantum dots are not limited to a specific approach. Remarkably, the emergence of the nodal hole states at the top of the valence band can be attributed to the formation of the orbital vortex textures through competition between the hole kinetic energy and the coupling to the conduction band states. We suggest an experimental test for our predictions of the reversed energy ordering and the existence of nodal ground states. We discuss how our findings and the studies of orbital textures could be also relevant for other materials systems.

cond-mat.mes-hall↗

arXiv2013

Retardation effects and the Coulomb pseudopotential in the theory of superconductivity

In the theory of electron-phonon superconductivity both the magnitude of the electron-phonon coupling $λ$ as well as the Coulomb pseudopotential $μ^*$ are important to determine the transition temperature $T_c$ and other properties. We calculate corrections to the conventional result for the Coulomb pseudopotential. Our calculation are based on the Hubbard-Holstein model, where electron-electron and electron-phonon interactions are local. We develop a perturbation expansion, which accounts for the important renormalization effects for the electrons, the phonons, and the electron-phonon vertex. We show that retardation effects are still operative for higher order corrections, but less efficient due to a reduction of the effective bandwidth. This can lead to larger values of the pseudopotential and reduced values of $T_c$. The conclusions from the perturbative calculations are corroborated up to intermediate couplings by comparison with non-perturbative dynamical mean-field results.

cond-mat.supr-con↗

arXiv2013

Energy dissipation in DC-field driven electron lattice coupled to fermion baths

Electron transport in electric-field-driven tight-binding lattice coupled to fermion baths is comprehensively studied. We reformulate the problem by using the scattering state method within the Coulomb gauge. Calculations show that the formulation justifies direct access to the steady-state bypassing the time-transient calculations, which then makes the steady-state methods developed for quantum dot theories applicable to lattice models. We show that the effective temperature of the hot-electron induced by a DC electric field behaves as $T_{\rm eff}=Cγ(Ω/Γ)$ with a numerical constant $C$, tight-binding parameter $γ$, the Bloch oscillation frequency $Ω$ and the damping parameter $Γ$. In the small damping limit $Γ/Ω\to 0$, the steady-state has a singular property with the electron becoming extremely hot in an analogy to the short-circuit effect. This leads to the conclusion that the dissipation mechanism cannot be considered as an implicit process, as treated in equilibrium theories. Finally, using the energy flux relation, we derive a steady-state current for interacting models where only on-site Green's functions are necessary.

cond-mat.mes-hall↗

arXiv2013

Solution of electric-field-driven tight-binding lattice in contact with fermion reservoir

Electrons in tight-binding lattice driven by DC electric field dissipate their energy through on-site fermionic thermostats. Due to the translational invariance in the transport direction, the problem can be block-diagonalized. We solve this time-dependent quadratic problem and demonstrate that the problem has an oscillatory steady-state. The steady-state occupation number shows that the Fermi surface disappears for any damping from the thermostats and any finite electric field. Despite the lack of momentum scattering, the conductivity takes the same form as the semi-classical Ohmic expression from the relaxation-time approximation.

cond-mat.str-el↗

arXiv2012

Theory of electron-phonon superconductivity: Does retardation really lead to a small Coulomb pseudopotential?

The theory of electron-phonon superconductivity depends on retardation drastically reducing effects of the strong Coulomb repulsion. The standard theory only treats the lowest order diagram, which is an uncontrolled approximation. We study retardation in the Hubbard-Holstein model in a controlled way using perturbation theory and dynamical mean-field theory. We calculate analytically second order results for the pseudopotential $μ^*$ and demonstrate the validity up to intermediate couplings by comparison with non-perturbative results. Retardation effects are still operative, but less efficient, leading to somewhat larger values of $μ^*$. Therefore, our theory can help to understand situations where the standard theory yields overestimates for $T_c$.

cond-mat.supr-con↗

arXiv2012

Imaginary-time quantum many-body theory out of equilibrium I: Formal equivalence to Keldysh real-time theory and calculation of static properties

We discuss the formal relationship between the real-time Keldysh and imaginary-time theory for nonequilibrium in quantum dot systems. The latter can be reformulated using the recently proposed Matsubara voltage approach. We establish general conditions for correct analytic continuation procedure on physical observables, and apply the technique to the calculation of static quantities in steady-state non-equilibrium for a quantum dot subject to a finite bias voltage and external magnetic field. Limitations of the Matsubara voltage approach are also pointed out.

cond-mat.str-el↗

arXiv2012

Imaginary-time quantum many-body theory out of equilibrium II: Analytic continuation of dynamic observables and transport properties

Within the imaginary-time theory for nonequilibrium in quantum dot systems the calculation of dynamical quantities like Green's functions is possible via a suitable quantum Monte-Carlo algorithm. The challenging task is to analytically continue the imaginary-time data for both complex voltage and complex frequency onto the real variables. To this end a function-theoretical description of dynamical observables is introduced and discussed within the framework of the mathematical theory of several complex variables. We construct a feasible maximum-entropy algorithm for the analytical continuation by imposing a continuity assumption on the analytic structure and provide results for spectral functions in stationary non-equilibrium and current-voltage characteristics for different values of the dot charging energy.

cond-mat.str-el↗

arXiv2011

Quantitative reliability of Migdal-Eliashberg theory for strong electron-phonon coupling

We reassess the validity of Migdal-Eliashberg (ME) theory for coupled electron-phonon systems for large couplings $λ$. Although model calculations have found that ME theory breaks down for $λ\sim 0.5$, it is routinely applied for $λ>1$ to strong coupling superconductors. To resolve this discrepancy it is important to distinguish between {\em bare} parameters, used as input in models, and {\em effective} parameters, derived from experiments. We show explicitly that ME gives accurate results for the critical temperature and the spectral gap for large effective $λ$. This provides quantitative theoretical support for the applicability of ME theory to strong coupling conventional superconductors.

cond-mat.supr-con↗