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Junren Shi

Publications and source records attributed to Junren Shi.

At least 73 records · Page 4Linked to original sources
arXiv2007

First-principles calculations of Cu(001) thin films: quantum size effect in surface energetics and surface chemical reactivities

First-principles calculations of Cu(001) free-standing thin films have been performed to investigate the oscillatory quantum size effects exhibited in surface energy, work function, atomic relaxation, and adsorption energy of the cesium adsorbate. The quantum well states have been shown and clarified at particular $k$-points corresponding to the stationary extrema in bulk Brillouin zone, which are in good agreement with experimental observations. The calculated surface energetics and geometry relaxations are clearly featured by quantum oscillations as a function of the film thickness of the film with oscillation periods characterized by a superposition of long and short length scales. Furthermore, we have investigated Cs adsorption onto Cu(001) thin films as a function of the film thickness. Our systematic calculated results clearly show the large-amplitude quantum oscillations in adsorption energetics, which may be used to tailor catalysis, chemical reactions and other surface processes in nanostructured materials.

cond-mat.mes-hall↗
arXiv2007

Theory of conserved spin current and its application to two dimensional hole gas

We present a detailed microscopic theory of the conserved spin current which is introduced by us [Phys. Rev. Lett. \textbf{96}, 196602 (2006)] and satisfies the spin continuity equation even for spin-orbit coupled systems. The spin transport coefficients $σ_{μν}^{s}$ as a response to the electric field are shown to consist of two parts, i.e., the conventional part $σ_{μν}^{s0}$ and the spin torque dipole correction $σ_{μν}^{sτ}$. As one key result, an Onsager relation between $σ_{μν}^{s}$ and other kinds of transport coefficients are shown. The expression for $σ_{μν}^{s}$ in terms of single-particle Bloch states are derived, by use of which we study the conserved spin Hall conductivity in the two dimensional hole gas modeled by a combined Luttinger and SIA Rashba spin-orbit coupling. It is shown that the two components in spin Hall conductivity usually have the opposite contributions. While in the absence of Rashba spin splitting, the spin Hall transport is dominated by the conventional contribution, the presence of Rashba spin splitting stirs up a large enhancement of the spin torque dipole correction, leading to an overall sign change for the total spin Hall conductivity. Furthermore, an approximate two-band calculation and the subsequent comparison with the exact four-band results are given, which reveals that the coupling between the heavy hole and light hole bands should be taken into account for strong Rashba spin splitting.

cond-mat.mes-hall↗
arXiv2006

Attractive electron-electron interaction induced by geometric phase in a Bloch band

We investigate electron pairing in the presence of the Berry curvature field that ubiquitously exists in ferromagnetic metals with spin-orbit coupling. We show that a sufficiently strong Berry curvature field on the Fermi surface can transform a repulsive interaction between electrons into an attractive one in the p-wave channel. We also reveal a topological possibility for turning an attractive s-wave interaction into one in the p-wave channel, even if the Berry curvature field only exists inside the Fermi surface (circle). We speculate that these novel mechanism might be relevant to the recently discovered ferromagnetic superconductors such as UGe$_{2}$ and URhGe.

cond-mat.supr-con↗
arXiv2006

Experimental Observation of the Inverse Spin Hall Effect at Room Temperature

We observe the inverse spin Hall effect in a two-dimensional electron gas confined in AlGaAs/InGaAs quantum wells. Specifically, we find that an inhomogeneous spin density induced by the optical injection gives rise an electric current transverse to both the spin polarization and its gradient. The spin Hall conductivity can be inferred from such a measurement through the Einstein relation and the Onsager relation, and is found to have the order of magnitude of $0.5(e^{2}/h)$. The observation is made at the room temperature and in samples with macroscopic sizes, suggesting that the inverse spin Hall effect is a robust macroscopic transport phenomenon.

cond-mat.mes-hall↗
arXiv2006

On A Proper Definition of Spin Current

The conventional definition of spin current is incomplete and unphysical in describing spin transport in systems with spin-orbit coupling. A proper and measurable spin current is established in this study, which fits well into the standard framework of near-equilibrium transport theory and has the desirable property to vanish in insulators with localized orbitals. Experimental implications of our theory are discussed.

cond-mat.mes-hall↗
arXiv2005

Berry phase correction to electron density of states in solids

Liouville's theorem on the conservation of phase space volume is violated by Berry phase in the semiclassical dynamics of Bloch electrons. This leads to a modification of the phase space density of states, whose significance is discussed in a number of examples: field modification of the Fermi-sea volume, connection to the anomalous Hall effect, and a general formula for orbital magnetization. The effective quantum mechanics of Bloch electrons is also sketched, where the modified density of states plays an essential role.

cond-mat.mes-hall↗
arXiv2005

Multiple Bosonic Mode Coupling in Electron Self-Energy of (La_2-xSr_x)CuO_4

High resolution angle-resolved photoemission spectroscopy data along the (0,0)-($π$,$π$) nodal direction with significantly improved statistics reveal fine structure in the electron self-energy of the underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ samples in the normal state. Fine structure at energies of (40$\sim$46) meV and (58$\sim$63)meV, and possible fine structure at energies of (23$\sim$29)meV and (75$\sim$85)meV, have been identified. These observations indicate that, in LSCO, more than one bosonic modes are involved in the coupling with electrons.

cond-mat.str-el↗
arXiv2004

A low temperature disordered phase of alpha-Pb/Ge(111)

A new structural phase transition has been observed at low temperatures for the one third of a monolayer (alpha phase) of Pb on Ge(111) using a variable-temperature scanning tunneling microscope. The well-known (r3xr3)R30 to (3x3) transition is accompanied by a new structural phase transition from (3x3) to a disordered phase at ~76 K. The formation of the new disordered phase is incompatible with the belief that the (3x3) phase is the ground state. The mechanism of the phase transition in this triangular lattice can be mapped onto antiferromagnetic spin Ising model, with the geometric frustration.

cond-mat.mtrl-sci↗
arXiv2004

Direct extraction of the Eliashberg function for electron-phonon coupling: A case study of Be(1010)

We propose a systematic procedure to directly extract the Eliashberg function for electron-phonon coupling from high-resolution angle-resolved photoemission data. The procedure is successfully applied to the Be(1010) surface, providing new insights to electron-phonon coupling at this surface. The method is shown to be robust against imperfections in experimental data and suitable for wider applications.

cond-mat.str-el↗
arXiv2003

Surface Phase Transitions Induced by Electron Mediated Adatom-Adatom Interaction

We propose that the indirect adatom-adatom interaction mediated by the conduction electrons of a metallic surface is responsible for the $\sqrt{3}\times \sqrt{3}\Leftrightarrow 3\times 3$ structural phase transitions observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction overwhelms the local stress field imposed by the substrate registry, the system suffers a phonon instability, resulting in a structural phase transition in the adlayer. Our theory is capable of explaining all the salient features of the $\sqrt{3}\times \sqrt{3}\Leftrightarrow 3\times 3$ transitions observed in Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of systems whose surfaces are metallic before the transition.

cond-mat.mtrl-sci↗
arXiv2003

Radiation-Induced "Zero-Resistance State" and the Photon Assisted Transport

We demonstrate that the radiation induced "zero-resistance state" observed in a two-dimensional electron gas is a result of the non-trivial structure of the density of states of the systems and the photon assisted transport. A toy model of a structureless quantum tunneling junction where the system has oscillatory density of states catches most of the important features of the experiments. We present a generalized Kubo-Greenwood conductivity formula for the photon assisted transport in a general system, and show essentially the same nature of the transport anomaly in a uniform system.

cond-mat.mes-hall↗
arXiv2003

Reply to Comment on: "Radiation-Induced 'Zero-Resistance State' and the Photon Assisted Transport"

We show that the comment by A.F. Volkov ignores a delicate issue in the conductance measurement for a hall bar system. In such system, $ρ_{xx}\approx ρ_{xy}^{2}σ_{xx}$ while $σ_{xy}\gg σ_{xx}$, as correctly pointed out in Ref.3. We clarify that the so called "zero resistance state" is actually a "zero conductance state". A discussion concerning the phase transition induced by the negative conductance is presented.

cond-mat.mes-hall↗
arXiv2001

Dephasing Effect in Photon-Assisted Resonant Tunneling through Quantum Dots

We analyze dephasing in single and double quantum dot systems. The decoherence is introduced by the Büttiker model with current conserving fictitious voltage leads connected to the dots. By using the non-equilibrium Green function method, we investigate the dephasing effect on the tunneling current. It is shown that a finite dephasing rate leads to observable effects. The result can be used to measure dephasing rates in quantum dots.

cond-mat.mes-hall↗
arXiv2001

The Droplet State and the Compressibility Anomaly in Dilute 2D Electron Systems

We investigate the space distribution of carrier density and the compressibility of two-dimensional (2D) electron systems by using the local density approximation. The strong correlation is simulated by the local exchange and correlation energies. A slowly varied disorder potential is applied to simulate the disorder effect. We show that the compressibility anomaly observed in 2D systems which accompanies the metal-insulator transition can be attributed to the formation of the droplet state due to disorder effect at low carrier densities.

cond-mat.str-el↗
arXiv2000

Dephasing and Metal-Insulator Transition

The metal-insulator transition (MIT) observed in two-dimensional (2D) systems is apparently contradictory to the well known scaling theory of localization. By investigating the conductance of disordered one-dimensional systems with a finite phase coherence length, we show that by changing the phase coherence length or the localization length, it is possible to observe the transition from insulator-like behavior to metal-like behavior, and the transition is a crossover between the quantum and classical regimes. The resemblance between our calculated results and the experimental findings of 2D MIT suggests that the observed metallic phase could be the result of a finite dephasing rate.

cond-mat.str-el↗
arXiv1999

A Droplet State in an Interacting Two-Dimensional Electron System

It is well known that the dielectric constant of two-dimensional (2D) electron system goes negative at low electron densities. A consequence of the negative dielectric constant could be the formation of the droplet state. The droplet state is a two-phase coexistence region of high density liquid and low density "gas". In this paper, we carry out energetic calculations to study the stability of the droplet ground state. The possible relevance of the droplet state to recently observed 2D metal-insulator transition is also discussed.

cond-mat.mes-hall↗