arXiv subjects

Z. X. Zhao

Publications and source records attributed to Z. X. Zhao.

At least 55 records · Page 3Linked to original sources

arXiv2004

The Dichotomy between Nodal and Antinodal Quasiparticles in Underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ Superconductors

High resolution angle-resolved photoemission measurements on underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ system show that, at energies below 70 meV, the quasiparticle peak is well defined around the ($π$/2,$π$/2) nodal region and disappears rather abruptly when the momentum is changed from the nodal point to the ($π$,0) antinodal point along the underlying ``Fermi surface''. It indicates that there is an extra low energy scattering mechanism acting upon the antinodal quasiparticles. We propose that this mechanism is the scattering of quasiparticles across the nearly parallel segments of the Fermi surface near the antinodes.

cond-mat.str-el↗

arXiv2003

$Fe_{0.5}Cu_{0.5}(Ba_{1-x}Sr_{x})_2YCu_2O_{7+δ}$ ($x$ = 0, 0.5 and 1) superconductors prepared by high pressure synthesis

In this paper, the $Fe$-containing superconductors $Fe_{0.5}Cu_{0.5}Ba_2YCu_2O_{7+δ}$, $Fe_{0.5}Cu_{0.5}BaSrYCu_2O_{7+δ}$ and $Fe_{0.5}Cu_{0.5}Sr_2YCu_2O_{7+δ}$ were successfully prepared by common solid-state reaction followed with a procedure of high pressure synthesis. The structural change and superconducting properties in $(Fe_xCu_{1-x})BaSrYCu_2O_{7+δ}$ ($x$ = 0 $\sim$ 1.0) systems were also investigated. Annealing experiments indicate that the occurrence of superconductivity in $Fe_{0.5}Cu_{0.5}(Ba_{1-x}Sr_{x})_2YCu_2O_{7+δ}$ ($x$ = 0, 0.5 and 1) systems is mainly induced by the procedure of high pressure synthesis, which causes the increase of oxygen content and the redistribution of $Fe$ atoms between $Cu(1)$ and $Cu(2)$ sites, but not from possible secondary phase of $YBa_2Cu_3O_{7-δ}$, $YBaSrCu_3O_{7-δ}$ or $YSr_2Cu_3O_{7-δ}$ superconductors.

cond-mat.supr-con↗

arXiv2003

The composite picture of the charge carriers in La2-xSrxCuO4 (0.063 < x < 0.11) superconductors

Through far-infrared studies of La2-xSrxCuO4 single crystals for x = 0.063, 0.07, 0.09, and 0.11, we found that only ~ 0.2 % of the total holes participated in the nearly dissipationless normal state charge transport and superconductivity. We have also observed characteristic collective modes at w ~ 18 cm-1 and 22 cm-1 due to the bound carriers in an electronic lattice (EL) state and the free carriers are massively screened by the EL. Our findings lead us to propose a composite picture of the charge system where the free carriers are coupled to and riding on the EL. This unique composite system of charge carriers may provide further insights into the understanding of the cuprate physics.

cond-mat.supr-con↗

arXiv2003

Anomalous superconducting properties at magic doping levels in under-doped La2-xSrxCuO4 single crystals

A series of high-quality under-doped La2-xSrxCuO4 superconductor crystals with x = 0.063 - 0.125 were prepared by traveling-solvent floating-zone (TSFZ) technique. We found by dc magnetic measurements that, in this series of crystals, the superconducting transition was quite sharp in the vicinity of the hole densities of x = 1/16 and x = 1/9 while it was much broader away from these two "magic numbers", and the Meissner fraction showed a remarkable minimum near x = 1/9. We concluded that these phenomena are reflections of intrinsic properties of this cuprate system. Our observations are discussed in light of recently proposed composite charge model together with charge inhomogeneity and electronic phase separations.

cond-mat.supr-con↗

arXiv2003

Correlation dynamics between electrons and ions in the fragmentation of D$_2$ molecules by short laser pulses

We studied the recollision dynamics between the electrons and D$_2^+$ ions following the tunneling ionization of D$_2$ molecules in an intense short pulse laser field. The returning electron collisionally excites the D$_2^+$ ion to excited electronic states from there D$_2^+$ can dissociate or be further ionized by the laser field, resulting in D$^+$ + D or D$^+$ + D$^+$, respectively. We modeled the fragmentation dynamics and calculated the resulting kinetic energy spectrum of D$^+$ to compare with recent experiments. Since the recollision time is locked to the tunneling ionization time which occurs only within fraction of an optical cycle, the peaks in the D$^+$ kinetic energy spectra provides a measure of the time when the recollision occurs. This collision dynamics forms the basis of the molecular clock where the clock can be read with attosecond precision, as first proposed by Corkum and coworkers. By analyzing each of the elementary processes leading to the fragmentation quantitatively, we identified how the molecular clock is to be read from the measured kinetic energy spectra of D$^+$ and what laser parameters be used in order to measure the clock more accurately.

physics.atom-ph↗

arXiv2003

Probing Molecular Dynamics at Attosecond Resolution with Femtosecond Laser Pulses

The kinetic energy distribution of D$^+$ ions resulting from the interaction of a femtosecond laser pulse with D$_2$ molecules is calculated based on the rescattering model. From analyzing the molecular dynamics, it is shown that the recollision time between the ionized electron and the D$_2^+$ ion can be read from the D$^+$ kinetic energy peaks to attosecond accuracy. We further suggest that more precise reading of the clock can be achieved by using shorter fs laser pulses (about 15fs).

physics.atom-ph↗

arXiv2003

The mechanical relaxation study of polycrystalline MgCNi3

The mechanical relaxation spectra of a superconducting and a non-superconducting MgCNi3 samples were measured from liquid nitrogen temperature to room temperature at frequency of kilohertz. There are two internal friction peaks (at 300 K labeled as P1 and 125 K as P2) for the superconducting sample. For the non-superconducting one, the position of P1 shifts to 250 K, while P2 is almost completely depressed. It is found that the peak position of P2 shifts towards higher temperature under higher measuring frequency. The calculated activation energy is 0.13eV. We propose an explanation relating P2 to the carbon atom jumping among the off-center positions. And further we expect that the behaviors of carbon atoms maybe correspond to the normal state crossovers around 150 K and 50 K observed by many other experiments.

cond-mat.supr-con↗

arXiv2003

Anisotropic vortex pinning in superconductors with a square array of rectangular submicron holes

We investigate vortex pinning in thin superconducting films with a square array of rectangular submicron holes ("antidots"). Two types of antidots are considered: antidots fully perforating the superconducting film, and "blind antidots", holes that perforate the film only up to a certain depth. In both systems, we observe a distinct anisotropy in the pinning properties, reflected in the critical current Ic, depending on the direction of the applied electrical current: parallel to the long side of the antidots or perpendicular to it. Although the mechanism responsible for the effect is very different in the two systems, they both show a higher critical current and a sharper IV-transition when the current is applied along the long side of the rectangular antidots.

cond-mat.supr-con↗

arXiv2003

Influence of Carbon Concentration on the Superconductivity in MgCxNi3

The influence of carbon concentration on the superconductivity (SC) in MgC$_{x}$Ni$_3$ has been investigated by measuring the low temperature specific heat combined with first principles electronic structure calculation. It is found that the specific heat coefficient $γ_n=C_{en}/T$ of the superconducting sample ($x\approx1$) in normal state is twice that of the non-superconducting one ($x\approx 0.85$). The comparison of measured $γ_n$ and the calculated electronic density of states (DOS) shows that the effective mass renormalization changes remarkably as the carbon concentration changes. The large mass renormalization for the superconducting sample and the low $T_{c}$(7K) indicate that more than one kind of boson mediated electron-electron interactions exist in MgC$_{x}$Ni$_3$.

cond-mat.supr-con↗

arXiv2003

Strictly In-Plane Vortex-like Excitations in Pseudogap Region of Underdoped $La_{2-x}Sr_xCuO_4$ Single Crystals

Nernst effect has been measured for an underdoped and an optimally doped $La_{2-x}Sr_xCuO_4$ single crystal with the magnetic field applied along different directions. For both samples, when $H \|$ c, a significant Nernst voltage appears above $T_c$ and keeps measurable up to a high temperature (100 K) as reported by Xu et al.[ Nature 406, 486(2000) ]. However, when $H \|$ a-b plane the Nernst signal drops below the noise level quickly above $T_c$ for the underdoped sample. Moreover, the in-plane Nernst data with field at different angles shows a nice scaling behavior with the c-axis component of the field. All these give strong indication for strictly in-plane vortex-like excitations in pseudo-gap region.

cond-mat.supr-con↗

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

Superconducting properties and c-axis superstructure of Mg1-xAlxB2

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with the significant change in inter-boron layer distance as reported previously by Slusky et al. Moreover,transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is essentially related to the structural transition. The modifications of superconducting transition temperature Tc, the normal state resistivity, and the upper critical field Bc2 by Al doping are discussed in terms of Al-substitution induced changes in the electronic structure at the Fermi energy.

cond-mat.supr-con↗

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

Superconducting Gap, Pseudogap and Their Relationship Investigated by Break-junction Tunneling Spectroscopy on $Bi_2Sr_2CaCu_2O_{8+δ}$ Single Crystals

Temperature and doping dependent tunneling spectroscopy on $Bi_2Sr_2CaCu_2O_{8+δ}$ single crystals has been investigated by using break-junction technique. The results provide direct evidence that the pseudogap persists into heavily overdoped regime, possibly in whole doping range of the superconducting phase. The gap value determined from the peak-to-peak of the conductance, $Δ_{p-p}$, exhibits a jump-like increase as $T$ is raised across the critical temperature. The magnitude of the jump, the pseudogap opening temperature and superconducting gap decrease with raising doping. The $Δ_{p-p}$ below $T_c$ can be naturally treated as the vector sum of two competing orders: a BCS-like superconducting gap and a residual pseudogap.

cond-mat.supr-con↗

arXiv2001

Effect of Mg and C contents in MgCNi3, and structure and superconductivity of MgCNi3-xCox

The effect of Mg and C contents on TC in MgCNi3, and structure and superconductivity of MgCNi3-xCox were studied. It is found that the excess of Mg and C in initial material mixture is favorable to improve TC and obtain single-phase samples. For preparing MgCNi3 superconductor, the optimum composition of starting materials is MgC1.45Ni3 with 20wt.% excess of Mg of the stoichiometric composition. In MgCNi3-xCox system, a continuous solid solution is formed, lattice parameter decreases slightly and TC decreases obviously with increasing x. A suppression of superconductivity is observed due to the substitution of Co (Mn) for Ni. The suppression effect is smaller for the substitution of Co than that of Mn.

cond-mat.supr-con↗

arXiv2001

Preparation and Superconductivity of a MgB2 Superconducting Tape

A MgB2 superconducting tape has been successfully prepared by a new way of boron diffusion into Mg tape. This tape superconductor has a Tc of 39 K with a sharp transition width of 0.4 K, high bulk critical current density jc of 6.7*10^5 A/cm^2 in zero field at 6 K, higher irreversibility line comparing to that of a high pressure synthesized bulk sample and very small crystalline grain with size in the scale of 10-200 nm.

cond-mat.supr-con↗

arXiv2001

Structural features, stacking faults, and grain boundaries in MgB2 superconducting materials

The structural properties of MgB2 superconductors have been analyzed by means of convergent-beam electron diffraction, high-resolution transmission-electron microscopy, and theoretical simulations. The MgB2 crystal has been identified to have a hexagonal structure with the space group of P6/mmm. Microstructural features of MgB6 materials depend evidently upon the synthesis pressures. Stacking faults and grain boundaries in a sample with the critical current density greater than 105A/cm2 have been extensively investigated.

cond-mat.supr-con↗

arXiv2001

Superconductivity and Aluminum Ordering in Mg1-xAlxB2

Superconductivity and structural properties of Mg1-xAlxB2 materials have been systematically investigated. Evident modifications in both superconductivity and microstructure are identified to originate from the Al ordering along the c-axis direction. The resultant superstructure phase has an optimal composition of MgAlB4 with the superconducting transition at around 12K. Brief diagrams illustrating the superconductivity and structural features of Mg1-xAlxB2 materials along with the increase of Al concentration are presented.

cond-mat.supr-con↗