arXiv subjects

Y. Z. Zhang

Publications and source records attributed to Y. Z. Zhang.

32 records · Page 2Linked to original sources

arXiv2005

Comments on "Vortex phase diagram of HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ thin films from magnetoresistance measurements"

We make comments on the paper presented by Kim \textit{et al.} [Phys. Rev. B \textbf{61}, 11317 (2000)]. The authors analyzed activation energies of HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ thin films with a scaling relation, and defined four vortex regions for thin films. We find that the definitions of four vortex regions and the scaling relation are questionable when studying their definition of the resistivity range for thermally activated flux flow. Using the empirical activation energy form suggested by Zhang \textit{et al.} [Phys. Rev. B \textbf{71}, 052502 (2005)] for lower resistivity data, we find that the form is not only in good agreement with the resistivity $ρ(T,H)$, but also with the apparent activation energy $-\partial \ln ρ(T,H)/\partial T^{-1}$.

cond-mat.supr-con↗

arXiv2005

Charge ordering in the spinels AlV$_2$O$_4$ and LiV$_2$O$_4$

We develop a microscopic theory for the charge ordering (CO) transitions in the spinels AlV$_2$O$_4$ and LiV$_2$O$_4$ (under pressure). The high degeneracy of CO states is lifted by a coupling to the rhombohedral lattice deformations which favors transition to a CO state with inequivalent V(1) and V(2) sites forming Kagomé and trigonal planes respectively. We construct an extended Hubbard type model including a deformation potential which is treated in unrestricted Hartree Fock approximation and describes correctly the observed first-order CO transition. We also discuss the influence of associated orbital order. Furthermore we suggest that due to different band fillings AlV$_2$O$_4$ should remain metallic while LiV$_2$O$_4$ under pressure should become a semiconductor when charge disproportionation sets in.

cond-mat.str-el↗

arXiv2004

Two dimensional scaling of resistance in flux flow region in $Tl_2Ba_2CaCu_2O_8$ thin films

The resistance of $Tl_2Ba_2CaCu_2O_8$ thin films has been measured when the angle between the applied fields and $ab$-plane of the film is changed continuously at various temperatures. Under various magnetic fields, the resistance can be well scaled in terms of the c-axis component of the applied fields at the same temperature in the whole angle range. Meanwhile, we show that the measurement of resistance in this way is a complementary method to determine the growth orientation of the anisotropic high-$T_c$ superconductors.

cond-mat.supr-con↗

arXiv2004

Metal-insulator transition in the quarter- filled frustrated checkerboard lattice

We study the electronic structure and correlations in the geometrically frustrated two dimensional checkerboard lattice. In the large U limit considered here we start from an extended Hubbard model of spinless fermions at half-filling. We investigate the model within two distinct Green's function approaches: In the first approach a single-site representation decoupling scheme is used that includes the effect of nearest neighbor charge fluctuations. In the second approach a cluster representation leading to a 'multiorbital' model is investigated which includes intra-cluster correlations exactly and those between clusters on a mean field basis. It is demonstrated that with increasing nearest-neighbor Coulomb interaction V both approaches lead to a metal-insulator transition with an associated 'Mott-Hubbard' like gap caused by V. Within the single site approach we also explore the possibility of charge order. Furthermore we investigate the evolution of the quasiparticle bands as funtion of V.

cond-mat.str-el↗

arXiv2004

Study of the ionic Peierls-Hubbard model using density matrix renormalization group methods

Density matrix renormalization group methods are used to investigate the quantum phase diagram of a one-dimensional half-filled ionic Hubbard model with bond-charge attraction, which can be mapped from the Su-Schrieffer-Heeger-type electron-phonon coupling at the antiadiabatic limit. A bond order wave (dimerized) phase which separates the band insulator from the Mott insulator always exists as long as electron-phonon coupling is present. This is qualitatively different from that at the adiabatic limit. Our results indicate that electron-electron interaction, ionic potential and quantum phonon fluctuations combine in the formation of the bond-order wave phase.

cond-mat.str-el↗

arXiv2004

Growth and superconducting transition of $Pr_{1-x} Ca_x Ba_2 Cu_3 O_{7-δ}$ ($x\approx0.5$) epitaxial thin films

Pr$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-δ}$($x\approx$0.5) thin films have been grown on SrTiO$_3$ and YSZ substrates by the pulsed laser ablation. The substrate temperature dependence of orientation and superconducting poperties were systematically studied. Good quality \textit{c} and {a}-axis orientated films can be obtained on SrTiO$_3$ via changing the substrate temperature solely. On YSZ, films with good \textit{c}-axis orientation can be grown, while it is hard to grow films with good \emph{a}-axis orientation by changing substrate temperature alone. The highest $T_{C0}$ is about 37K, which is found in the films grown on YSZ with a good \emph{c}-axis orientation. For the films grown on STO, however, the highest $T_{C0}$ is about 35.6K with a mixed orientation of \emph{c}-axis and \emph{a}-axis. In most of the superconducting films, the weak temperature dependence of the normal state resistivity, as characterized by small $R(290K)/R(50K)\leq2$ ratios, together with a weak localization behavior just above $T_C$ could be attributed to the essential scattering due to the localized electronic states. The superconducting transitions in a field up to 10 T along \textit{c}-axis have been measured on a \textit{c}-axis oriented film grown on SrTiO$_3$. The zero-temperature in-plane upper critical field $B^{ab}_{C2}$(0) is estimated from the resistivity transition data.

cond-mat.supr-con↗

arXiv2004

Multidimensional inhomogeneous cosmology in scalar tensor theory

Exact cosmological solutions are obtained for a five dimensional inhomogeneous fluid distribution along with a Brans-Dicke type of scalar field. The set includes varied forms of matter field including $ρ+p=0$, where p is the 3D isotropic pressure. Depending on the signature of 4-space curvature our solutions admit of indefinite expansion in the usual 3-space and dimensional reduction of the fifth dimension. Due to the presence of the scalar field the case $p=-ρ$ does not yield an exponential expansion of the scale factor, which strikingly differs from our earlier investigations without scalar field.The \emph{effective} four dimensional values of entropy and matter are calculated and possible consequences of entropy and matter generation in the 4D world as a result of dimensional reduction of the extra space are also discussed. Encouraging to point out that aside from the well known big bang singularity our inhomogeneous cosmology is spatially regular everywhere. Further our model seems to suggest an alternative mechanism pointing to a smooth pass over from a primordial, inhomogeneous cosmological phase to a 4D homogeneous one.

gr-qc↗

arXiv2004

Dimerization in a half-filled one-dimensional extended Hubbard model

We use a density matrix renormalization group method to study quantitatively the phase diagram of a one-dimensional extended Hubbard model at half-filling by investigating the correlation functions and structure factors. We confirm the existence of a novel narrow region with long-rang bond-order-wave order which is highly controversial recently between the charge-density-wave phase and Mott insulator phase. We determined accurately the position of the tricritical point $U_t\simeq 7.2t$, $V_t\simeq 3.746t$ which is quite different from previous studies.

cond-mat.str-el↗

arXiv2003

New Upper Limit of Terrestrial Equivalence Principle Test for Rotating Extended Bodies

Improved terrestrial experiment to test the equivalence principle for rotating extended bodies is presented, and a new upper limit for the violation of the equivalence principle is obtained at the level of 1.6$% \times 10^{\text{-7}}$, which is limited by the friction of the rotating gyroscope. It means the spin-gravity interaction between the extended bodies has not been observed at this level.

hep-ph↗

arXiv2002

Quantum tunneling of vortices in a MgB$_2$ superconductor

Magnetic relaxation in a MgB$_2$ superconductor was measured. The temperature dependence of the normalized relaxation rate was determined for three different magnetic fields. By extrapolating these rates to T=0 K, we find that these extrapolations do not approach zero, indicating quantum tunneling of vortices in the superconductor. A quantum correction of the relaxation rate, followed by the correction of the magnetic moment, is proposed. Using the quantum correction, we find that $U_0$ increases with decreasing temperature and approaches a maximum at T=0.

cond-mat.supr-con↗

arXiv2002

Effect of Earth's rotation on the trajectories of free-fall bodies in Equivalence Principle Experiment

Owing to Earth's rotation a free-fall body would move in an elliptical orbit rather than along a straight line forward to the center of the Earth. In this paper on the basis of the theory for spin-spin coupling between macroscopic rotating bodies we study violation of the equivalence principle from long-distance free-fall experiments by means of a rotating ball and a non-rotating sell. For the free-fall time of 40 seconds, the difference between the orbits of the two free-fall bodies is of the order of 10^{-9}cm which could be detected by a SQUID magnetometer owing to such a magnetometer can be used to measure displacements as small as 10^{-13} centimeters.

gr-qc↗

arXiv2002

Irreversibility line of the MgB$_{2}$ superconductor

Magneto-resistivity of a c-axis oriented MgB$_{2}$ thin film was studied in perpendicular and parallel magnetic fields up to $\sim $23 T with temperatures down to 0.38 K. Resistive critical magnetic fields were determined. Large separations between irreversibility lines and upper critical magnetic fields are observed. An effective quantum parameter as a function of temperature is proposed to explain the separations under the theoretical framework of quantum fluctuations, in good agreement with our experimental and previously published experimental data.

cond-mat.supr-con↗

arXiv2001

Null Result for the Violation of Equivalence Principle with Free-Fall Rotating Gyroscopes

The differential acceleration between a rotating mechanical gyroscope and a non-rotating one is directly measured by using a double free-fall interferometer, and no apparent differential acceleration has been observed at the relative level of 2x10{-6}. It means that the equivalence principle is still valid for rotating extended bodies, i.e., the spin-gravity interaction between the extended bodies has not been observed at this level. Also, to the limit of our experimental sensitivity, there is no observed asymmetrical effect or anti-gravity of the rotating gyroscopes as reported by hayasaka et al.

gr-qc↗

arXiv2001

A dimerized spin fluid in a one-dimensional electron system

The ground state of a one-dimensional Hubbard model with a bond-charge attraction W term at half-filling is investigated by the density matrix renormalization group method. It is confirmed that the spin gap will be closed at U>8W. But the long-range bond order wave survives even when the spin gap is closed. It indicates that the ground state is a novel dimerized spin fluid at U>8W. By a charge-spin transformation, it is shown that there should be a dimerized metallic phase at U<-8W. Furthermore, it is found that the Hubbard interaction U enhances initially the dimerization for a weak bond charge attraction W whereas it reduces monotonously the dimerization for a stronger bond charge attraction W.

cond-mat.str-el↗