arXiv subjects

T. K. Lee

Publications and source records attributed to T. K. Lee.

At least 37 records · Page 2Linked to original sources

arXiv2005

Comparative analysis of specific heat of YNi2B2C using nodal and two-gap models

The magnetic field dependence of low temperature specific heat in YNi2B2C was measured and analyzed using various pairing order parameters. At zero magnetic field, the two-gap model which has been successfully applied to MgB2 and the point-node model, appear to describe the superconducting gap function of YNi2B2C better than other models based on the isotropic s-wave, the d-wave line nodes, or the s+g wave. The two energy gaps, delta_L=2.67 meV and delta_S=1.19 meV are obtained. The observed nonlinear field dependence of electronic specific heat coefficient, gamma(H)~H0.47, is quantitatively close to gamma(H)~H0.5 expected for nodal superconductivity or can be qualitatively explained using two-gap scenario. Furthermore, the positive curvature in Hc2(T) near Tc is qualitatively similar to that in the other two-gap superconductor MgB2.

cond-mat.supr-con↗

arXiv2005

Spin dynamics in the antiferromagnetic phase for electron-doped cuprate superconductors

Based on the $t$-$t'$-$t''$-$J$ model we have calculated the dynamical spin susceptibilities in the antiferromagnetic (AF) phase for electron-doped cuprates, by use of the slave-boson mean-field theory and random phase approximation. Various results for the susceptibilities versus energy and momentum have been shown at different dopings. At low energy, except the collective spin-wave mode around $(π,π)$ and 0, we have primarily observed that new resonance peaks will appear around $(0.3π,0.7π)$ and equivalent points with increasing doping, which are due to the particle-hole excitations between the two AF bands. The peaks are pronounced in the transverse susceptibility but not in the longitudinal one. These features are predicted for neutron scattering measurements.

cond-mat.supr-con↗

arXiv2004

Study of gossamer superconductivity and antiferromagnetism in the t-J-U model

The d-wave superconductivity (dSC) and antiferromagnetism are analytically studied in a renormalized mean field theory for a two dimensional t-J model plus an on-site repulsive Hubbard interaction $U$. The purpose of introducing the $U$ term is to partially impose the no double occupancy constraint by employing the Gutzwiller approximation. The phase diagrams as functions of doping $δ$ and $U$ are studied. Using the standard value of $t/J=3.0$ and in the large $U$ limit, we show that the antiferromagnetic (AF) order emerges and coexists with the dSC in the underdoped region below the doping $δ\sim0.1$. The dSC order parameter increases from zero as the doping increases and reaches a maximum near the optimal doping $δ\sim0.15$. In the small $U$ limit, only the dSC order survives while the AF order disappears. As $U$ increased to a critical value, the AF order shows up and coexists with the dSC in the underdoped regime. At half filing, the system is in the dSC state for small $U$ and becomes an AF insulator for large $U$. Within the present mean field approach, We show that the ground state energy of the coexistent state is always lower than that of the pure dSC state.

cond-mat.supr-con↗

arXiv2004

Absence of Coexistence of Superconductivity and Antiferromagnetism of the Hole-Doped Two-Dimensional Extended $t-J$ Model

The possibility of coexistence of superconductivity (SC) and antiferromagnetic long range order (AFLRO) of the two-dimensional extended $t-J$ model in the very underdoped region is studied by the variational Monte-Carlo (VMC) method. In addition to using previously studied wave functions, a recently proposed new wave function generated from the half-filled Mott insulator is used. For hole-doped systems, the phase boundary between AFLRO and $d-$wave SC for the physical parameters, $J/t=0.3$, $t'/t=-0.3$ and $t''/t=0.2$, is located near hole density $δ_c = 0.06$, and there is {\it no} coexistence. The phase transition is first-order between these two homogeneous phases at $δ_c$.

cond-mat.str-el↗

arXiv2004

Orbital Polarization, Surface Enhancement and Quantum Confinement in Nano-cluster Magnetism

Within a rather general tight-binding framework, we studied the magnetic properties of Ni$_{n}$ clusters with $n=9$ through 60. In addition to usual hopping, exchange, and spin-orbit coupling terms, our Hamiltonian also included orbital correlation and valence orbital shift of surface atoms. We show that orbital moment not only contributes appreciably to the total moment in this range of cluster size, but also dominates the oscillation of total moment with respect to the cluster size. Surface enhancement is found to occur not only for spin but, even stronger, also for orbital moment. The magnitude of this enhancement depends mainly on the coordination deficit of surface atoms, well described by a simple interpolation. For very small clusters ($n \leq 20$), quantum confinement of 4s electrons has drastic effects on 3$d$ electron occupation, and thus greatly influences both spin and orbital magnetic moments. With physically reasonable parameters to account for orbital correlation and surface valence orbital shift, our results are in good quantitative agreement with available experiments, evidencing the construction of a unified theoretical framework for nano-cluster magnetism.

cond-mat.mtrl-sci↗

arXiv2004

Fermi surface evolution in the antiferromagnetic state for the electron-doped t-t'-t''-J model

By use of the slave-boson mean-field approach, we have studied the electron-doped t-t'-t''-J model in the antiferromagnetic (AF) state. It is found that at low doping the Fermi surface (FS) pockets appear around $(\pmπ,0)$ and $(0,\pmπ)$, and upon increasing doping the other ones will form around $(\pm{π\over 2},\pm{π\over 2})$. The evolution of the FS with doping as well as the calculated spectral weight are consistent with the experimental results.

cond-mat.supr-con↗

arXiv2004

Enhancement of Pairing Correlation by t' in the Two-Dimensional Extende d t-J Model

We investigate the effects of the next-nearest-neighbor ($t'$) and the third-nearest-neighbor (t") hopping terms on superconductivity (SC) correlation in the 2D hole-doped extended t-J model based on the variational Monte-Carlo (VMC), mean-field (MF) calculation, and exact diagonalization (ED) method. Despite of the diversity of the methods employed, the results all point to a consistent conclusion: While the d-wave SC correlation is slightly suppressed by t' and t" in underdoped regions, it is greatly enhanced in the optimal and overdoped regions. The optimal T_c is a result upon balance of these two opposite trends.

cond-mat.supr-con↗

arXiv2003

Low-energy spectra in the t-J type models at light doping

Based on a variational approach, we propose that there are two kinds of low energy states in the t-J type models at low doping. In a quasi-particle state an unpaired spin bound to a hole with a well-defined momentum can be excited with spin waves. The resulting state shows a suppression of antiferromagnetic order around the hole with the profile of a {\em spin bag}. These spin-bag states with spin and charge or hole separated form a continuum of low-energy excitations. Very different properties predicted by these two kinds of states explain a number of anomalous results observed in the exact diagonalization studies on small clusters up to 32 sites.

cond-mat.str-el↗

arXiv2003

Guided Simulated Annealing Method for Optimization Problems

Incorporating the concept of order parameter of the mean-field theory into the simulated annealing method, we presented a new optimization algorithm, the guided simulated annealing method. In this method mean-field order parameters are calculated to guide the configuration search for the global minimum. Allowing fluctuations and improvement of mean-field values iteratively, this method successfully identified global minima for several difficult optimization problems. Application of this method to the HP lattice-protein model has found a new lowest energy state for an $N = 100$ sequence that was not found by other methods before. Results for spin glass models are also presented.

cond-mat.stat-mech↗

arXiv2002

Theory for Slightly Doped Antiferromagnetic Mott Insulators

Trial wavefunctions, constructed explicitly from the unique 2-dimensional Mott insulating state with antiferromagnetic order, are proposed to describe the low-energy states of a Mott insulator slightly doped with holes or electrons. With the state behaving like charged quasi-particles with well-defined momenta, a rigid band is observed. These states have much less pairing correlations than previously studied ones. Small Fermi patches obtained are consistent with recent experiments on high Tc cuprates doped lightly with holes or electrons. States showing the incoherent and spin-bag behaviors are also discussed. Using these wavefunctions, a number of results obtained by exact calculations are reproduced.

cond-mat.str-el↗

arXiv2002

Magnetic polarization induced by nonmagnetic impurities in high Tc cuprates

The magnetic polarization induced by nonmagnetic impurities such as Zn in high Tc cuprate compounds is studied by the variational Monte Carlo simulation. The variational wave function is constructed from the eigenstates obtained from Bogoliubov de Gennes mean field Hamiltonian for the two-dimensional t-J model. A Jastrow factor is introduced to account for the induced magnetic moment and the repulsion between holes and the impurity. A substantial energy gain is obtained by forming an antiferromagnetic polarization covering 4 or 5 lattice sites around the impurity. We also found the doping dependence for the induced magnetic moment consistent with experiments.

cond-mat.str-el↗

arXiv2001

Revisit Phase Separation of the Two-Dimensional $t-J$ model by the Power-Lanczos Method

The power-Lanczos (PL) method is one kind of Green's function Monte-Carlo simulation, which is improved by Lanczos iterations. The ground state energies of strongly-correlated models can be evaluated by this method quite accurately. In this report, the boundary of phase separation (PS) of the two-dimensional $t-J$ model is investigated by the power-Lanczos method and Maxwell construction. The energies are compared with the results evaluated by other methods. Our conclusion is that there is no phase separation for $J/t \le 0.4$.

cond-mat.str-el↗

arXiv2001

Superconducting condensation energy and neutron resonance in high-Tc superconductors

Based on the slave-boson theory of the two-dimensional $t-t'-J$ model, we calculate the superconducting condensation energy for optimally doped and overdoped high $T_{c}$ cuprates at finite temperatures using a renormalized random phase approximation. The contributions come from the mean-field part and the antiferromagnetic spin fluctuations. In the presence of neutron resonance peak at $(π,π)$, the latter is shown to have similar temperature and doping dependences as the difference in antiferromagnetic (AF) exchange energy between the normal and the superconducting state. This difference has been proposed to be related to the superconducting condensation energy by Scalapino and White. The total condensation energy, however, is about 1/2 smaller than the proposed AF exchange energy difference and shows a more rapid decrease as the temperature rises. The doping dependence of the condensation energy is found to be consistent with experiments. In particular, near zero temperature, our result shows a good quantitative agreement with experiments.

cond-mat.supr-con↗

arXiv2000

Quasiparticle Resonant States Induced by a Unitary Impurity in a d-Wave Superconductor

The quasiparticle resonant states around a single nonmagnetic impurity with unitary scattering in a d-wave superconductor is studied by solving the Bogoliubov-de Gennes equations based on a t-J model. Both the spatial variation of the order parameter and the local density of states (LDOS) around the impurity have been investigated. We find: (i) A particle-hole symmetric system has a single symmetric zero-energy peak in the LDOS regardless of the size of the superconducting coherence length ξ_0; (ii) For the particle-hole asymmetric case, an asymmetric splitting of the zero-energy peak is intrinsic to a system with a small value of k_{F}ξ_0.

cond-mat.supr-con↗

arXiv1999

A Consistent Picture for Resonance Neutron Peak and ARPES Spectra in High-Tc Superconductors

The spectra observed in ARPES measurements are examined together with the resonance peak observed in neutron scatterings, based on the slave-boson approach to $t-t^{\prime}-J$ model. We show that the peak/dip/hump features arise from the scattering of electrons by collective spin excitations which, at the same time, give rise to the neutron resonance mode. The doping dependences and the dispersions of the peak/dip/hump positions are shown to be consistent with experiments. Our results indicate that recently observed $\cos (6θ)$ deviation from the pure d-wave also result from the renormalization by spin fluctuations.

cond-mat.supr-con↗

arXiv1999

Numerical Study of the Pairing Correlation of the t-J Type Models

We reported that the pair-pair correlation function of the two-dimensional t-J model does not have long-range d-wave superconducting correlations in the interesting parameter range of $J/t \leq 0.5$. The power-Lanczos method is used under the assumption of monotonic behavior. This assumption has been well checked in the two-dimensional t-J and attractive Hubbard model. Here we re-examine this criterion of monotonic behavior of the pairing correlation function for the one-dimensional and two-leg t-J ladder where other accurate numerical results are available. The method seems to be working well.

cond-mat.str-el↗

arXiv1999

Stripe Stability in the Extended t-J Model on Planes and Four-Leg Ladders

The tendencies to phase-separation and stripe formation of the t-J model on planes and four-leg ladders have been here reexamined including hole hopping terms t', t'' beyond nearest-neighbor sites. The motivation for this study is the growing evidence that such terms are needed for a quantitative description of the cuprates. Using a variety of computational techniques it is concluded that the stripe tendencies considerably weaken when experimentally realistic t'<0, t''>0 for hole-doped cuprates are considered. However, a small t'>0 actually enhances the stripe formation.

cond-mat.str-el↗

arXiv1999

Effect of the Fermi surface destruction on transport properties in underdoped cuprates

Motivated by recent experimental measurements on the Fermi surface(FS) destruction in underdoped high-$T_{c}$ cuprates, we examine its effect on the transport properties based on the Boltzmann equation approach. The effect is modeled by simply taking the density of states for electrons in the gapped regions to be zero. Within the nearly antiferromagnetic Fermi liquid model, we calculate the temperature dependences of the dc resistivity, the inverse Hall angle and the Hall coefficient. It is shown that the effect of the FS destruction on transport properties is sensitive to the existance and the range of the flat band near $(0,\pmπ)$ in the dispersion of electrons, and the anistropy of the relaxation rate along the Fermi surface. We find that the experimental data are better described by the cold spot model, i.e., the transports are determined mainly by the contribution of the electrons near the Brillouin-zone diagonals.

cond-mat.supr-con↗