arXiv subjects

T. Xiang

Publications and source records attributed to T. Xiang.

At least 91 records · Page 5Linked to original sources

arXiv2002

Low-frequency spins and the ground state in high-Tc cuprates

We study the spin fluctuations of pure and Zn-substituted high temperature superconductors (HTS) using the muon spin relaxation (muSR) technique. Superconductivity is found to coexist with low frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by muSR decreases with increasing doping and vanishes above a critical doping pc=0.19 where the normal state pseudogap vanishes. Furthermore, it is at pc that our penetration depth studies show an abrupt change in the doping dependence of the superfluid density. For p>pc the absolute value of the superfluid density is large and nearly independent of carrier concentration whereas for p<pc it drops rapidly, signalling a crossover to weak superconductivity. These results are discussed in terms of a quantum transition involving fluctuating short-range magnetic order that separates the superconducting phase diagram of HTS into two distinct ground states.

cond-mat.supr-con↗

arXiv2002

Microscopic Hamiltonian for Zn or Ni substituted high temperature cuprate superconductors

We have derived the effective low energy Hamiltonian for Zn or Ni substituted high-T_c cuprates from microscopic three-band models consisting of the most relevant Cu or impurity 3d and O 2p orbitals. We find that both scattering potential and hopping integral induced by impurities have a finite range but decay very fast with distance from the impurity. The Zn scattering potential is very strong and attractive for electrons. The Ni scattering potential is much weaker than the Zn case, resulting from the hybridization between Ni ions and O holes. This profound difference is due to neither the electric charge nor d-level location, but rather because of the interplay between the valence state of the impurity and the strong correlation background. It gives a natural account for the unusual effect of Ni and Zn on the reduction of superconducting transition temperature. The interlayer hopping of electrons is highly anisotropic and nonlocal, determined by the in-plane electronic structure. This leads to a quantum interference of states from different sites and affects strongly the scanning tunneling spectrum perpendicular to CuO_2 planes.

cond-mat.supr-con↗

arXiv2002

Evidence for a generic novel quantum transition in high-Tc cuprates

We study the low-energy spin fluctuations and superfluid density of a series of pure and Zn-substituted high-$T_c$ superconductors (HTS) using the muon spin relaxation and $ac$-susceptibility techniques. At a critical doping state, $p_c$, we find (i) simultaneous abrupt changes in the magnetic spectrum in the superconducting ground state and (ii) that the slowing down of spin fluctuations becomes singular at T=0. These results provide experimental evidence for a novel quantum transition that separates the superconducting phase diagram of HTS into two distinct ground states.

cond-mat.supr-con↗

arXiv2001

The magnetic field dependence of the electronic specific heat of Y_{0.8}Ca_{0.2}Ba_2Cu_3O_{6+x}

We have measured the electronic specific heat of $Y_{0.8}Ca_{0.2}Ba_2Cu_3O_{7-δ}$ using a high-resolution differential technique from liquid helium temperature to room temperature in an applied magnetic field up to 13T. The field dependence of the electronic specific heat at low temperatures in the superconducting state behaves differently in the overdoped and underdoped regimes, varing as $\sqrt{H}$ in the overdoped regime but as $H$ in the underdoped regime. An entropy loss is observed in the normal state in optimal and underdoped samples, which can be attributed to the opening of a normal state psuedogap. From the temperature and field dependences of the free energy, the temperature dependence of the upper critical field $H_{c2}$ is determined. For the overdoped sample ($x=0.79$), we find $H_{c2}$ to have a negative curvature.

cond-mat.supr-con↗

arXiv2001

Critical Behaviour in the Spin Fluctuations and Superfluid Density of La$_{2-x}$Sr$_x$CuO$_4$

We study the doping dependence of low frequency spin fluctuations and the zero-temperature superfluid density of La$_{2-x}$Sr$_x$CuO$_4$ using the muon spin relaxation ($μ$SR) and ac-susceptibility techniques. Superconductivity is found to coexist with low frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by $μ$SR decreases with increasing $x$ and vanishes above a critical doping $x_c\sim 0.19$. This value of $x_c$ coincides with the doping at which the normal state pseudogap extrapolates to zero. The superfluid density behaves in the opposite way to the low frequency spin fluctuations. It increases with $x$ and becomes nearly doping-independent for $x>x_c$. These results are consistent with predictions involving quantum criticality at $x_c$.

cond-mat.supr-con↗

arXiv2001

Penetration Depth Measurements in MgB_2: Evidence for Unconventional Superconductivity

We have measured the magnetic penetration depth of the recently discovered binary superconductor MgB_2 using muon spin rotation and low field $ac$-susceptibility. From the damping of the muon precession signal we find the penetration depth at zero temperature is about 85nm. The low temperature penetration depth shows a quadratic temperature dependence, indicating the presence of nodes in the superconducting energy gap.

cond-mat.supr-con↗

arXiv2000

Universal c-axis conductivity of high-$T_c$ oxides in the superconducting state

The anisotropy in the temperature dependence of the in-plane and c-axis conductivities of high-T_c cuprates in the superconducting state is shown to be consistent with a strong in-plane momentum dependence of both the quasiparticle scattering rate and the interlayer hopping integral. Applying the cold spot scattering model recently proposed by Ioffe and Millis (Phys. Rev. B {\bf 58}, 11631 (1998)) to the superconducting state, we find that the c-axis conductivity varies approximately as T^3 in an intermediate temperature regime, in good agreement with the experimental result for optimally doped YBa_2Cu_3O_{7-x} and Bi_2Sr_2CaCu_2O_{8+x}.

cond-mat.supr-con↗

arXiv1999

Effects of doping on thermally excited quasiparticles in the high-$T_c$ superconducting state

The physical properties of low energy superconducting quasiparticles in high- $T_c$ superconductors are examined using magnetic penetration depth and specific heat experimental data. We find that the low energy density of states of quasiparticles of La$_{2-x}$Sr$_x$CuO$_4$ scales with $(x-x_c)/T_c$ to the leading order approximation, where $x_c $ is the critical doping concentration below which $T_c=0$. The linear temperature term of the superfluid density is renormalized by quasiparticle interactions and the renormalization factor times the Fermi velocity is found to be doping independent.

cond-mat.supr-con↗

arXiv1999

c-Axis Superfluid Response and Pseudogap in High-Tc Superconductors

To gain insight into the out-of-plane electrodynamics of high-$T_c$ superconduct ors we have measured the absolute values and temperature dependence of the c-a xis magnetic penetration depth $λ_c(T)$ for two typical single layer high -$T_c$ cuprates, La$_{2-x}$Sr$_x$CuO$_4$ and HgBa$_2$CuO$_{4+x}$ as a function o f doping. A distinct change in the behaviour of $λ_c$ is observed near 0. 20 holes/Cu, which is related to the opening of the normal state pseudogap. The variation of $λ_c$ with doping is qualitatively similar to that of the in -plane component, $λ_{ab}$, which suggests that the c-axis superfluid r esponse is mainly governed by the superconducting condensation energy. The stron g doping dependence of $λ_c(0)$ for p<0.20 provides an explanation for the discrepancies in the literature.

cond-mat.supr-con↗

arXiv1998

Low Temperature Superfluid Response of High-Tc Superconductors

We have reviewed our theoretical and experimental results of the low temperature superfluid response function of high temperature superconductors (HTSC). In clean high-Tc materials the in-plane superfluid density rho_s^{ab} varies linearly with temperature. The slope of this linear T term is found to scale approximately with 1/Tc which, according to the weak coupling BCS theory for a d-wave superconductor, implies that the gap amplitude scales approximately with Tc. A T^5 behavior of the out-of-plane superfluid density rho_s^c for clean tetragonal HTSC was predicted and observed experimentally in the single layer Hg-compound HgBa_2CuO_{4+delta}. In other tetragonal high-Tc compounds with relatively high anisotropy, such as Hg_2Ba_2Ca_2Cu_3O_{8+delta}, rho_s^c varies as T^2 due to disorder effects. In optimally doped YBa_2Cu_3O_{7-delta}, rho_s^c varies linearly with temperature at low temperatures, but in underdoped YBa_2Cu_3O_{7-delta}, rho_s^c varies as T^2 at low temperatures; these results are consistent with our theoretical calculations.

cond-mat.supr-con↗

arXiv1997

Systematic behaviour of the in-plane penetration depth in d-wave cuprates

We report the temperature T and oxygen concentration dependences of the penetration depth of grain-aligned YBa_2Cu_3O_{7-δ} with δ= 0.0, 0.3 and 0.43. The values of the in-plane λ_{ab}(0) and out-of-plane λ_{c}(0) penetration depths, the low temperature linear term in λ_{ab}(T), and the ratio λ_{c}(0) /λ_{ab}(T) were found to increase with increasing $δ$. The systematic changes of the linear term in λ_{ab}(T) with T_c found here and in recent work on HgBa_2Ca_{n-1} Cu_nO_{2n+2+δ} (n = 1 and 3) are discussed.

cond-mat.supr-con↗

arXiv1996

Applications of the density matrix renormalisation group to problems in magnetism

We review White's density matrix renormalisation group method, an increasingly popular method for the solution of low dimensional quantum Hamiltonians. We describe some applications to frustrated spin systems, quantum critical phenomena, two dimensional classical and one dimensional quantum systems at non-zero temperature, and low energy properties of two dimensional quantum models such as the Hubbard and Heisenberg Hamiltonians.

cond-mat↗

arXiv1996

Density matrix renormalisation group for a quantum spin chain at non-zero temperature

We apply a recent adaptation of White's density matrix renormalisation group (DMRG) method to a simple quantum spin model, the dimerised $XY$ chain, in order to assess the applicabilty of the DMRG to quantum systems at non-zero temperature. We find that very reasonable results can be obtained for the thermodynamic functions down to low temperatures using a very small basis set. Low temperature results are found to be most accurate in the case when there is a substantial energy gap.

cond-mat↗

arXiv1996

c axis superfluid response of Copper-Oxide superconductors

A novel interplay between d-wave superconducting order parameter symmetry and the underlying Cu 3d orbital based electronic structure of Copper-Oxides leads to a striking anisotropy in the superfluid response of these systems. In clean tetragonal materials the c-axis penetration depth increases as T^5 at low temperature T, in contrast to linear T behavior in the ab-plane. Disorder is a relevant perturbation which causes all components of the superfluid response to depend quadratically on temperature at low temperature. However, the cross-over temperature scale from the intrinsic d-wave behavior to the disorder dominated behavior for the in-plane response may be different from that for the out-plane response.

cond-mat↗

arXiv1996

Quasiparticle energy dispersion in doped two dimensional quantum antiferromagnets

The quasiparticle dispersion in the one-hole t-t'-t"-J model is studied. Both the finite-size diagonalization and the self-consistent Born approximation calculations have been performed and compared. The quasiparticle band structures in the hole and electron doped high-Tc cuprates are qualitatively different. In the hole doped compounds, the band maxima locate at (pi/2 , pi/2), while in the electron doped compounds the band maxima locate at (pi, 0) and its equivalent points. The angle-resolved photoemission data for the quasiparticle dispersion of Sr_2CuO_2Cl_2 can be quantitatively reproduced using the one band t-t'-t"-J model with the three-site hopping term.

cond-mat↗

arXiv1996

Density-matrix renormalization-group method in momentum space

A momentum-space approach of the density-matrix renormalization-group (DMRG) method is developed. Ground state energies of the Hubbard model are evaluated using this method and compared with exact diagonalization as well as quantum Monte-Carlo results. It is shown that the momentum-space DMRG is a very useful numerical tool for studying the Hubbard model and other fundamental models of interacting electrons in two dimensions. For the Hubbard model in two dimensions, the momentum-space DMRG method reproduces accurately the exact diagonalization results of ground state energies on a $4\times 4$ lattice and yields new upper bounds of ground state energies on an 8$\times$8 lattice.

cond-mat↗

arXiv1995

Numerical and approximate analytical results for the frustrated spin-1/2 quantum spin chain

We study the $T=0$ frustrated phase of the $1D$ quantum spin-$\frac 12$ system with nearest-neighbour and next-nearest-neighbour isotropic exchange known as the Majumdar-Ghosh Hamiltonian. We first apply the coupled-cluster method of quantum many-body theory based on a spiral model state to obtain the ground state energy and the pitch angle. These results are compared with accurate numerical results using the density matrix renormalisation group method, which also gives the correlation functions. We also investigate the periodicity of the phase using the Marshall sign criterion. We discuss particularly the behaviour close to the phase transitions at each end of the frustrated phase.

cond-mat↗

arXiv1995

Superfluid anisotropy in YBCO: Evidence for pair tunneling superconductivity

Proximity effect and pair tunneling models are applied as alternative scenarios to explain the recently measured $ab$-plane and $c$-axis components of the superfluid tensor in Copper-Oxide superconductors which contain chains, such as $YBa_2Cu_3O_{6.95}$. It is argued that conventional proximity effect models, which couple chains and planes via single electron tunneling only, are incompatible with the experimental observations. On the other hand several surprising features of the experimental data are readily explained by the presence of a microscopic pair tunneling process.

cond-mat↗