arXiv subjects

Degang Zhang

Publications and source records attributed to Degang Zhang.

25 records · Page 2Linked to original sources

arXiv2006

Exact Landau Levels in Two-Dimensional Electron Systems with Rashba and Dresselhaus Spin-Orbit Interactions in a Perpendicular Magnetic Field

We study a two-dimensional electron system in the presence of both Rashba and Dresselhaus spin-orbit interactions in a perpendicular magnetic field. Defining two suitable boson operators and using the unitary transformations we are able to obtain the exact Landau levels in the range of all the parameters. When the strengths of the Rashba and Dresselhaus spin-orbit interactions are equal, a new analytical solution for the vanishing Zeeman energy is found, where the orbital and spin wave functions of electron are separated. It is also shown that in this case the Zeeman and spin-orbit splittings are independent of the Landau level index $n$. Due to the Zeeman energy, new crossing between the eigenstates $|n, k, s=1, σ>$ and $|n+1, k, s^\prime =-1, σ^\prime>$ is produced at certain magnetic field for larger Rashba spin-orbit coupling. This degeneracy leads to a resonant spin Hall conductance if it happens at the Fermi level.

cond-mat.mes-hall↗

arXiv2004

Impurity-induced Local Density of States in a D-wave Superconductor Carrying a Supercurrent

The local density of states (LDOS) and its Fourier component induced by a unitary impurity in a supercurrent-carrying d-wave superconductor are investigated. Both of these quantities possess a reflection symmetry about the line passing through the impurity site and along the supercurrent if it is applied along the antinodal or nodal direction. With increasing supercurrent, both the coherence and resonant peaks in the LDOS are suppressed and slightly broadened. Under a supercurrent along the antinodal direction, the coherence peaks split into double peaks. The modulation wavevectors associated with elastic scatterings of quasiparticles by the defect from one constant-energy piece of the Fermi surface to another are displayed as bright or dark spots in the Fourier space of the LDOS image, and they may be suppressed or enhanced, and shifted depending on the applied current and the bias voltage.

cond-mat.supr-con↗

arXiv2003

Conductance Characteristics between a Normal Metal and a Superconductor Carrying a Supercurrent

The low-temperature conductance (G) characteristics between a normal metal and a clean superconductor (S) carrying a supercurrent $I_s$ parallel to the interface is theoretically investigated. Increasing $I_s$ causes lowering and broadening of (1) coherence peaks of s-wave S, and d-wave S at (100) contact, (2) midgap-states-induced zero-bias conductance peak for d-wave S at (110) contact, and (3) Andreev-reflection-induced enhancement of $G$ within the gap near the metallic-contact limit. Novel features found include a current-induced central peak and a three-humped structure at intermediate barrier strength, etc.

cond-mat.supr-con↗

arXiv2003

The Energy-dependent Checkerboard Patterns in Cuprate Superconductors

Motivated by the recent scanning tunneling microscopy (STM) experiments [J. E. Hoffman {\it et al.}, Science {\bf 297}, 1148 (2002); K. McElroy {\it et al.}, Nature (to be published)], we investigate the real space local density of states (LDOS) induced by weak disorder in a d-wave superconductor. We first present the energy dependent LDOS images around a single weak defect at several energies, and then point out that the experimentally observed checkerboard pattern in the LDOS could be understood as a result of quasiparticle interferences by randomly distributed defects. It is also shown that the checkerboard pattern oriented along $45^0$ to the Cu-O bonds at low energies would transform to that oriented parallel to the Cu-O bonds at higher energies. This result is consistent with the experiments.

cond-mat.supr-con↗

arXiv2002

Non-Kramers degeneracy and oscillatory tunnel splittings in the biaxial Spin System

We have investigated analytically quantum tunneling of large spin in the biaxial spin system with the magnetic field applied along the hard and medium anisotropy axes by using a purely quantum-mechanical approach. When the magnetic field parallels the hard axis, the tunnel splittings of all the energy level pairs are oscillatory as a function of the magnetic field. The quenching points are completely determined by the coexistence of solutions of the Ince's equation. When the magnetic field points the medium axis, the tunnel splitting oscillations disappear due to no coexistence of solutions. These results coincide the recent experimental observations in the nanomagnet Fe$_8$.

cond-mat.mes-hall↗

arXiv2002

Charge Modulations in the Superconducting State of the Cuprates

Motivated by the recent scanning tunneling microscopy (STM) and neutron scattering experiments, we investigate various charge density wave orders coexisting with superconductivity in the cuprate superconductors. The explicit expressions of the local density of states and its Fourier component at the ordering wavevector for the weak charge modulations are derived. It is shown that the STM experiments in $Bi_2Sr_2CaCu_2O_{8+δ}$ cannot be explained by a site- or bond-centered charge modulation alone, but agree well with the presence of the dimerization hopping and transverse pairing modulations. We also calculate the spectral function for the charged stripes, which is measured by the ARPES experiments.

cond-mat.supr-con↗

arXiv2002

Energy-Dependent LDOS Modulation in Cuprate Superconductors

Motivated by the recent scanning tunneling microscopy (STM) experiment [J. E. Hoffman, {\it et al.}, Science {\bf 297}, 1148 (2002)], we investigate the energy-dependent modulation induced by a weak and extended defect in a d-wave superconductor. The Fourier component of the local density of states (LDOS) is calculated up to the first order in the defect parameters. Our numerically obtained image maps together with the energy-dependent charge modulation wave vectors at different dopings exhibit the essential features as those measured by the experiment. We also predict new modulation wave vectors in the first Brillouin zone. Hopefully, they could be verified by future STM experiments.

cond-mat.supr-con↗