arXiv subjects

Z. Q. Yang

Publications and source records attributed to Z. Q. Yang.

13 recordsLinked to original sources

arXiv2020

Fastest Frozen Temperature for a Thermodynamic System

For a thermodynamic system obeying both the equipartition theorem in high temperature and the third law in low temperature, the curve showing relationship between the specific heat and the temperature has two common behaviors:\ it terminates at zero when the temperature is zero Kelvin and converges to a constant as temperature is higher and higher. Since it is always possible to find the characteristic temperature $T_{C}$ to mark the excited temperature as the specific heat almost reaches the equipartition value, it is reasonable to find a temperature in low temperature interval, complementary to $T_{C}$. The present study reports a possibly universal existence of the such a temperature $\vartheta$, defined by that at which the specific heat falls \textit{fastest} along with decrease of the temperature. For the Debye model of solids, above the temperature $\vartheta$ the Debye's law starts to fail.

cond-mat.stat-mech↗

arXiv2019

No existence of the geometric potential for a Dirac fermion on two-dimensional curved surfaces of revolution

For a free particle that non-relativistically moves on a curved surface, there are curvature-induced quantum potentials that significantly influence the surface quantum states, but the experimental results in topological insulators, whenever curved or not, indicate no evidence of such a potential, implying that there does not exist such a quantum potential for the relativistic particles, constrained on the surface or not. Within the framework of Dirac quantization scheme, we demonstrate a general result that for a Dirac fermion on a two-dimensional curved surface of revolution, no curvature-induced quantum potential is permissible.

cond-mat.mes-hall↗

arXiv2019

General covariant geometric momentum, gauge potential and a Dirac fermion on a two-dimensional sphere

For a particle that is constrained on an ($N-1$)-dimensional ($N\geq2$) curved surface, the Cartesian components of its momentum in $N$-dimensional flat space is believed to offer a proper form of momentum for the particle on the surface, which is called the geometric momentum as it depends on the mean curvature. Once the momentum is made general covariance, the spin connection part can be interpreted as a gauge potential. The present study consists in two parts, the first is a discussion of the general framework for the general covariant geometric momentum. The second is devoted to a study of a Dirac fermion on a two-dimensional sphere and we show that there is the generalized total angular momentum whose three cartesian components form the $su(2)$ algebra, obtained before by consideration of dynamics of the particle, and we demonstrate that there is no curvature-induced geometric potential for the fermion.

hep-th↗

arXiv2019

Multi-outcome homodyne detection in a coherent-state light interferometer

The Cramér-Rao bound plays a central role in both classical and quantum parameter estimation, but finding the observable and the resulting inversion estimator that saturates this bound remains an open issue for general multi-outcome measurements. Here we consider multi-outcome homodyne detection in a coherent-light Mach-Zehnder interferometer and construct a family of inversion estimators that almost saturate the Cramér-Rao bound over the whole range of phase interval. This provides a clue on constructing optimal inversion estimators for phase estimation and other parameter estimation in any multi-outcome measurement.

quant-ph↗

arXiv2010

Density-functional studies of spin-orbit splitting in graphene on metals

Spin-orbit splitting in graphene on Ni, Au, or Ag (111) substrates was examined on the basis of density-functional theory. Graphene grown on the three metals was found to have Rashba splitting of a few or several tens of meV. The strong splitting obtained on Au or Ag substrates was mainly ascribed to effective hybridization of graphene $p_{z}$ state with Au or Ag $d_{z^{2}}$ states, rather than charge transfer as previously proposed. Our work provides theoretical understandings of the metal-induced Rashba effect in graphene.

cond-mat.mes-hall↗

arXiv2010

Electronic and magnetic properties of bilayer graphene with intercalated adsorption atoms C, N and O

We present an ab-initio density function theory to investigate the electronic and magnetic structures of the bilayer graphene with intercalated atoms C, N, and O. The intercalated atom although initially positioned at the middle site of the bilayer interval will finally be adsorbed to one graphene layer. Both N and O atoms favor the bridge site (i.e. above the carbon-carbon bonding of the lower graphene layer), while the C atom prefers the hollow site (i.e. just above a carbon atom of the lower graphene layer and simultaneously below the center of a carbon hexagon of the upper layer). Concerning the magnetic property, both C and N adatoms can induce itinerant Stoner magnetism by introducing extended or quasilocalized states around the Fermi level. Full spin polarization can be obtained in N-intercalated system and the magnetic moment mainly focuses on the N atom. In C-intercalated system, both the foreign C atom and some carbon atoms of the bilayer graphene are induced to be spin-polarized. N and O atoms can easily get electrons from carbon atoms of bilayer graphene, which leads to Fermi level shifting downward to valence band and thus producing the metallic behavior in bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2008

Novel electronic structure induced by a highly strained oxide interface with incommensurate crystal fields

The misfit oxide, Bi$_{2}$Ba$_{1.3}$K$_{0.6}$Co$_{2.1}$O$_{y}$, made of alternating rocksalt-structured [BiO/BaO] layers and hexagonal CoO$_{2}$ layers, was studied by angle-resolved photoemission spectroscopy. Detailed electronic structure of such a highly strained oxide interfaces is revealed for the first time. We found that under the two incommensurate crystal fields, electrons are confined within individual sides of the interface, and scattered by umklapp scattering of the crystal field from the other side. In addition, the high strain on the rocksalt layer raises its chemical potential and induces large charge transfer to the CoO$_{2}$ layer. Furthermore, a novel interface effects, the interfacial enhancement of electron-phonon interactions, is discovered. Our findings of these electronic properties lay a foundation for designing future functional oxide interfaces.

cond-mat.str-el↗

arXiv2007

Spin filtering implemented through Rashba and weak magnetic modulations

We present two theoretical schemes for spin filters in one-dimensional semiconductor quantum wires with spatially modulated Rashba spin-orbit coupling (SOC) as well as weak magnetic potential. For case I, the SOC is periodic and the weak magnetic potential is applied uniformly along the wire. Full spin polarizations with opposite signs are obtained within two separated energy intervals. For case II, the weak magnetic potential is periodic while the SOC is uniform. An ideal negative/positive switching effect for spin polarization is realized by tuning the strength of SOC. The roles of SOC, magnetic potential, and their coupling on the spin filtering are analyzed.

cond-mat.mes-hall↗

arXiv2006

Ideal switching effect in periodic spin-orbit coupling structures

An ideal switching effect is discovered in a semiconductor nanowire with a spatially-periodic Rashba structure. Bistable `ON' and `OFF' states can be realized by tuning the gate voltage applied on the Rashba regions. The energy range and position of `OFF' states can be manipulated effectively by varying the strength of the spin-orbit coupling (SOC) and the unit length of the periodic structure, respectively. The switching effect of the nanowire is found to be tolerant of small random fluctuations of SOC strength in the periodic structure. This ideal switching effect might be applicable in future spintronic devices.

cond-mat.mes-hall↗

arXiv2006

High-energy scale revival and giant kink in the dispersion of a cuprate superconductor

In the present photoemission study of a cuprate superconductor Bi1.74Pb0.38Sr1.88CuO6+delta, we discovered a large scale dispersion of the lowest band, which unexpectedly follows the band structure calculation very well. The incoherent nature of the spectra suggests that the hopping-dominated dispersion occurs possibly with the assistance of local spin correlations. A giant kink in the dispersion is observed, and the complete self-energy containing all interaction information is extracted for a doped cuprate in the low energy region. These results recovered significant missing pieces in our current understanding of the electronic structure of cuprates.

cond-mat.supr-con↗

arXiv2006

Realization of arbitrary single-qubit gates through control of spin-orbit couplings in semiconductor nanowires

We propose a theoretical scheme to realize arbitrary single-qubit gates through two simple device units: one-dimensional semiconductor wires with Dresselhaus spin-orbit coupling (SOC) and Rashba SOC, separately. Qubit information coded by the electron spin can be accurately manipulated by the SOC when crossing the semiconductor wire. The different manipulative behaviors in Dresselhaus and Rashba wires enable us to make the diverse quantum logic gates. Furthermore, by connecting the Dresselhaus and Rashba units in series, we obtain a universal set of single qubit gates: Hadamard, phase, and $π/8$ gates, inferring that an arbitrary single qubit gate can be achieved. Because the total transmission is satisfied in the two device units, all the logic gates we have obtained are lossless. In addition, a ballistic spintronic switch is proposed in the present investigation.

cond-mat.mes-hall↗

arXiv2002

Disorder-induced melting of the charge order in thin films of Pr0.5Ca0.5MnO3

We have studied the magnetic-field-induced melting of the charge order in thin films of Pr0.5Ca0.5MnO3 (PCMO) films on SrTiO3 (STO) by X-ray diffraction, magnetization and transport measurement. At small thickness (25 nm) the films are under tensile strain and the low-temperature melting fields are of the order of 20 T or more, comparable to the bulk value. With increasing film thickness the strain relaxes, which leads to a strong decrease of the melting fields. For a film of 150 nm, with in-plane and out-of-plane lattice parameters closer to the bulk value, the melting field has reduced to 4 T at 50 K, with a strong increase in the hysteretic behavior and also an increasing fraction of ferromagnetic material. Strain relaxation by growth on a template of YBa2Cu3O(7-delta) or by post-annealing yields similar results with an even stronger reduction of the melting field. Apparently, strained films behave bulk-like. Relaxation leads to increasing suppression of the CO state, presumably due to atomic scale disorder produced by the relaxation process.

cond-mat.mtrl-sci↗

arXiv2001

Strain release of (La,Ca)MnO3 thin films by YBa2Cu3O7-d

La1-xCaxMnO3 (x = 0.3; LCMO) films of different thickness were sputter-deposited on SrTiO3(100) (STO) single crystal substrates with and without YBa2Cu3O7-d (YBCO) as template layer. The electric and magnetic properties and the microstructure of the films were studied. Clear differences in the out-of-plane lattice parameter and the temperature of the metal-insulator transition show that the YBCO buffer layer is very effective in relaxing the strain of the LCMO films, which is quite difficult to release in LCMO films directly deposited on STO. Use of such buffer layers for strain release may prove to be a quite general tool.

cond-mat.mtrl-sci↗