arXiv subjects

Qingshan Yuan

Publications and source records attributed to Qingshan Yuan.

At least 19 recordsLinked to original sources

arXiv2007

Interface-mediated pairing in field effect devices

We consider the pairing induced in a strictly 2D electron gas (2DEG) by a proximate insulating film with polarizable localized excitations. Within a model of interacting 2D electrons and localized two-level systems, we calculate the critical temperature $T_c$ as a function of applied voltage and for different materials properties. Assuming that a sufficient carrier density can be induced in a field-gated device, we argue that superconductivity may be observable in such systems. $T_c$ is found to be a nonmonotonic function of both electric field and the excitation energy of the two-level systems.

cond-mat.str-el↗

arXiv2006

s-Wave-Like excitation in the superconducting state of electron-doped cuprates with d-wave pairing

An intrinsic physical mechanism, based on the doping evolution of the Fermi surface (FS), is explored to reconcile the contradictory experimental results on the superconducting (SC) pairing symmetry in electron-doped cuprates. It is argued that the FS pocket around $(π/2,π/2)$ has not yet formed until doping reaches about the optimal value. Therefore, in the underdoped regime, even if the SC order parameter is $d$ wave which vanishes along the line $k_x=k_y$, the quasiparticle excitation gap is still finite and looks s-wave-like due to the absence of the FS across that line. This makes it possible, with d-wave SC pairing, to understand those experiments which evidenced the s-wave quasiparticle excitation. An explicit theory with consideration of both antiferromagnetic and SC orders is implemented to exhibit the FS evolution from underdoping to overdoping and associated with it the variations of the quasiparticle property, electronic density of states, and low temperature dependences of the physical quantities heat capacity and superfluid density.

cond-mat.supr-con↗

arXiv2006

Theory of antiferromagnetism in the electron-doped cuprate superconductors

On the basis of the Hubbard model, we present the formulation of antiferromagnetism in electron-doped cuprates using the fluctuation-exchange approach. Taking into account the spin fluctuations in combination with the impurity scattering effect due to the randomly distributed dopant-atoms, we investigate the magnetic properties of the system. It is shown that the antiferromagnetic transition temperature, the onset temperature of the pseudogap formation, the single particle spectral density, and the staggered magnetization obtained by the present approach are in very good agreement with the experimental results. The distribution function in momentum space at very low temperature is observed to differ significantly from that of the Fermi liquid. Also, we find zero-energy peak in the density of states (DOS) of the antiferromagnetic phase. This DOS peak is sharp in the low doping regime, and disappears near the optimal doping where the AF order becomes weak.

cond-mat.str-el↗

arXiv2005

Nonmonotonic gap in the coexisting antiferromagnetic and superconducting state for electron-doped cuprate superconductors

We argue that the experimentally observed nonmonotonic gap in electron-doped cuprates at optimal doping is the lowest quasiparticle excitation energy in the coexisting antiferromagnetic (AF) and superconducting (SC) state. The idea is implemented by studying the coexistence of AF and SC orders with the t-t'-t''-J model. Although the pairing gap itself is assumed to be the simplest $d$ wave which is monotonic, we have found that the quasiparticle excitation gap in the coexisting state is nonmonotonic, with the maxima around the hot spots where the Fermi surface is missing due to the AF gap. Within the same framework of the coexisting state the spectral function is also calculated at optimal doping. The obtained results are all consistent with experiments.

cond-mat.supr-con↗

arXiv2005

Doping dependence of the electron-doped cuprate superconductors from the antiferromagnetic properties of the Hubbard model

Within the Kotliar-Ruckenstein slave-boson approach, we have studied the antiferromagnetic (AF) properties for the $t$-$t'$-$t''$-$U$ model applied to electron-doped cuprate superconductors. Due to inclusion of spin fluctuations the AF order decreases with doping much faster than obtained in the Hartree-Fock theory. Under an intermediate {\it constant} $U$ the calculated doping evolution of the spectral intensity has satisfactorily reproduced the experimental results, without need of a strongly doping-dependent $U$ as argued earlier. This may reconcile a discrepancy suggested in recent studies on photoemission and optical conductivity.

cond-mat.str-el↗

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

Comment on ``Strain effect and the phase diagram of La$_{1-x}$Ba$_x$MnO$_3$ thin films''

Recent experiments in La$_{1-x}$Ba$_x$MnO$_3$ thin films by Zhang {\it et al.} {[Phys. Rev. B {\bf 64}, 184404 (2001)]} showed that the ferromagnetic Curie temperature $T_c$ is enhanced (reduced) by tensile (compressive) strain. The results are regarded to be anomalous because it is generally understood by the authors that the tensile strain should lead to a reduction of the electron hopping $t$ (and thus also $T_c$) due to an elongation of the (in-plane) Mn-O bond length. In this comment such a general understanding of the strain effects is disapproved. It is argued that the tensile strain leads primarily to a larger Mn-O-Mn bond angle (with the Mn-O bond length nearly unchanged), thus the pure geometric consideration on $t$ is already in the correct direction towards the experimental findings by Zhang {\it et al.} At the same time, an indirect strain effect under consideration of the dynamic electron-phonon coupling is proposed, which may improve the quantitative explanation.

cond-mat.str-el↗

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↗

arXiv2003

Impurity effects on s+g-wave superconductivity in borocarbides Y(Lu)Ni_2B_2C

Recently a hybrid s+g-wave pairing is proposed to describe the experimental observation for a nodal structure of the superconducting gap in borocarbide YNi$_2$B$_2$C and possibly LuNi$_2$B$_2$C. In this paper the impurity effects on the s+g-wave superconductivity are studied in both Born and unitarity limit. The quasiparticle density of states and thermodynamics are calculated. It is found that the nodal excitations in the clean system are immediately prohibited by impurity scattering and a finite energy gap increases quickly with the impurity scattering rate. This leads to an activated behavior in the temperature dependence of the specific heat. Qualitative agreement with the experimental results is shown. Comparison with d-wave and some anisotropic s-wave studied previously is also made.

cond-mat.supr-con↗

arXiv2003

BCS theory for s+g-wave superconductivity borocarbides Y(Lu)Ni$_2$B$_2$C

The s+g mixed gap function Δ_k=Δ{[(1-x)-x\sin^4θ\cos4ϕ]} (x: weight of g-wave component) has been studied within BCS theory. By suitable consideration of the pairing interaction, we have confirmed that the coexistence of s- and g-wave, as well as the state with equal s and g amplitudes (i.e., x=1/2) may be stable. This provides the semi-phenomenological theory for the s+g-wave superconductivity with point nodes which has been observed experimentally in borocarbides YNi_2B_2C and possibly in LuNi_2B_2C.

cond-mat.supr-con↗

arXiv2002

Charge ordering in half-doped Pr(Nd)$_{0.5}$Ca$_{0.5}$MnO$_3$ under magnetic field

Recent experiments in Pr(Nd)$_{0.5}$Ca$_{0.5}$MnO$_3$ thin films exhibited (multiple) reentrant charge ordering (CO) transitions with change of {\it temperature} T under fixed magnetic field H, which are in contrast to the results for the corresponding bulk materials. To explain the experimental findings, a model including the double-exchange mechanism, intersite Coulomb interaction and electron-phonon coupling is proposed, for which the reentrant CO is naturally obtained due to the temperature dependence of the band-type coherent polaron hopping. Various results for the CO in the (H, T) plane are extensively discussed. The theory is considered to be valid for both thin films and bulk materials.

cond-mat.str-el↗

arXiv2001

A study of Peierls instabilities for a two-dimensional t-t' model

In this paper we study Peierls instabilities for a half-filled two-dimensional tight-binding model with nearest-neighbour hopping $t$ and next nearest-neighbour hopping $t'$ at zero and finite temperatures. Two dimerization patterns corresponding to the same phonon vector $(π, π)$ are considered to be realizations of Peierls states. The effect of imperfect nesting introduced by $t'$ on the Peierls instability, the properties of the dimerized ground state, as well as the competition between two dimerized states for each $t'$ and temperature $T$, are investigated. It is found: (i). The Peierls instability will be frustrated by $t'$ for each of the dimerized states. The Peierls transition itself, as well as its suppression by $t'$, may be of second- or first-order. (ii). When the two dimerized states are considered jointly, one of them will dominate the other depending on parameters $t'$ and $T$. Two successive Peierls transitions, that is, the system passing from the uniform state to one dimerized state and then to the other take place with decrease of temperature for some $t'$ values. Implications of our results to real materials are discussed.

cond-mat.str-el↗

arXiv2001

Spin-Peierls transition in an anisotropic two-dimensional XY model

The two-dimensional Jordan-Wigner transformation is used to investigate the zero temperature spin-Peierls transition for an anisotropic two-dimensional XY model in adiabatic limit. The phase diagram between the dimerized (D) state and uniform (U) state is shown in the parameter space of dimensionless interchain coupling $h$ $(=J_{\perp}/J)$ and spin-lattice coupling $η$. It is found that the spin-lattice coupling $η$ must exceed some critical value $η_c$ in order to reach the D phase for any finite $h$. The dependence of $η_c$ on $h$ is given by $-1/\ln h$ for $h\to 0$ and the transition between U and D phase is of first-order for at least $h>10^{-3}$.

cond-mat.str-el↗

arXiv2001

Coexistence of bond-order wave and antiferromagnetism in a two-dimensional half-filled Peierls-Hubbard model

The two-dimensional Peierls-Hubbard model is studied at half-filling within both Hartree-Fork and Kotliar-Ruckenstein slave-boson theory. The interplay between two types of long-range order, bond-order wave (BOW) and antiferromagnetism (AFM), is analysed for two representative dimerization patterns, corresponding both to the same wavevector $(π,π)$. For each pattern, the Peierls dimerization (and associated BOW) is weakened and finally suppressed with increasing Hubbard on-site interaction, and correspondingly AFM is gradually enhanced. In particular, a coexistence regime with both BOW and AFM order is established in the parameter space of electron-lattice and Hubbard interactions.

cond-mat.str-el↗

arXiv2001

Imperfect nesting and Peierls instability for a two-dimensional tight-binding model

Based on a half-filled two-dimensional tight-binding model with nearest-neighbour and next nearest-neighbour hopping the effect of imperfect Fermi surface nesting on the Peierls instability is studied at zero temperature. Two dimerization patterns corresponding to a phonon vector $(π, π)$ are considered. It is found that the Peierls instability will be suppressed with an increase of next nearest-neighbour hopping which characterizes the nesting deviation. First and second order transitions to a homogeneous state are possible. The competition between the two dimerized states is discussed.

cond-mat.str-el↗

arXiv2001

The Néel order for a frustrated antiferromagnetic Heisenberg model: beyond linear spin-wave theory

Within Dyson-Maleev (DM) transformation and self-consistent mean-field treatment, the Néel order/disorder transition is studied for an antiferromagnetic Heisenberg model which is defined on a square lattice with a nearest neighbour exchange $J_1$ and a next-nearest neighbour exchange $J_2$ along only one of the diagonals. It is found that the Néel order may exist up to $J_2/J_1=0.572$, beyond its classically stable regime. This result qualitatively improves that from linear spin-wave theory based on Holstein-Primakoff transformation.

cond-mat.str-el↗

arXiv2000

Phase diagram of a generalized Hubbard model applied to orbital order in manganites

The magnetic phase diagram of a two-dimensional generalized Hubbard model proposed for manganites is studied within Hartree-Fock approximation. In this model the hopping matrix includes anisotropic diagonal hopping matrix elements as well as off-diagonal elements. The antiferromagnetic (AF), ferromagnetic (F), canted (C) and paramagnetic (P) states are included in the analysis as possible phases. It is found that away from half-filling only the canted and F states may exist and AF and P states which are possible for the usual Hubbard model do not appear. This is because the F order has already developed for on-site repulsion U=0 due to the hopping matrix of the generalized model. When applied for manganites the orbital degree is described by a pseudospin. Thus our ``magnetic'' phase diagram obtained physically describes how orbital order changes with $U$ and with doping for manganites. Part of our results are consistent with other numerical calculations and some experiments.

cond-mat.str-el↗