Searcharxiv⌕ Search

arXiv subjects

Zhao-Bin Su

Publications and source records attributed to Zhao-Bin Su.

At least 37 records · Page 2Linked to original sources
arXiv1999

Gauge field theory of transport and magnetic relaxation in underdoped cuprates

Based on recently proposed U(1)xSU(2) Chern-Simons gauge field theory, an interpretation of the transport and magnetic relaxation properties of underdoped cuprates is proposed, taking into account the short range antiferromagnetic order. The interplay of the doping-dependent spin gap (explicitly derived by us) effect and dissipation due to gauge fluctuations gives rise to a crossover from metallic to insulating behavior of conductivity as temperature decreases, in semi-quantitative agreement with experimental data. For the same reason the magnetic relaxation rate shows a maximum nearby. Various crossover temperatures related to spin gap effects are shown to be different manifestations of the same energy scale.

cond-mat.str-el↗
arXiv1999

Schwinger boson mean field theory of the Heisenberg Ferrimagnetic Spin Chain

The Schwinger boson mean field theory is applied to the quantum ferrimagnetic Heisenberg chain. There is a ferrimagnetic long range order in the ground state. We observe two branches of the low lying excitation and calculate the spin reduction, the gap of the antiferromagnetic branch, and the spin fluctuation at $T=0K$. These results agree with the established numerical results quite well. At finite temperatures, the long range order is destroyed because of the disappearance of the Bose condensation. The thermodynamic observables, such as the free energy, magnetic susceptibility, specific heat, and the spin correlation at $T>0K$, are calculated. The $Tχ_{uni}$ has a minimum at intermediate temperatures and the spin correlation length behaves as $T^{-1}$ at low temperatures. These qualitatively agree with the numerical results and the difference is small at low temperatures.

cond-mat.str-el↗
arXiv1999

The Effective Theory of Hole Doped Spin-1 Chain

An effective theory for the hole doped spin-1 antiferromagnetic chain is proposed in this paper. The two branches of low energy quasipaticle excitation is obtained by the diagrammic technique. In the large t limit(in which t is the hole hopping term), the lower band is essentially the bound state of one hole and one magnon and the other band is the sigle hole state. We find a critical value of t, $t_c=0.21Δ_H$(in which $Δ_H$ is the Haldane gap).For $t>t_c$, with the decrement of t, the mixing of these two bands become stronger and stronger, and at the same time the effective band mass becomes larger and larger. When $t<t_c$ the minimum of the lower band moves away from zero the another point between zero and $π/2$. The spin strcture factor is also calculated in this paper, and we find that for large t limit the main contribution is from the inter-band transition which induce a resonant peak in the Haldane gap. While for small t limit the main contribution is from the intra-band transition which only cause a diffusion like broad bump in the Haldan gap.

cond-mat.str-el↗
arXiv1998

U(1)xSU(2) Chern--Simons gauge theory of underdoped cuprate superconductors

The Chern-Simons bosonization with U(1)xSU(2) gauge field is applied to 2-D t-J model in the limit t >> J, to study the normal state properties of underdoped cuprate superconductors. We prove the existence of an upper bound on the partition function for holons in a spinon background, and we find the optimal spinon configuration saturating the upper bound on average--a coexisting flux phase and s+id-like RVB state. After neglecting the feedback of holon fluctuations on the U(1) field B and spinon fluctuations on the SU(2) field V, the holon field is a fermion and the spinon field is a hard--core boson. We show that the B field produces a πflux phase for holons, converting them into Dirac--like fermions, while the V field, taking into account the feedback of holons produces a gap for spinons vanishing in zero doping limit. The nonlinear sigma-model with a mass term describes the crossover from short-ranged antiferromagnetic (AF) state in doped samples to long range AF order in reference compounds. Moreover, we derive a low--energy effective action in terms of spinons, holons and a self-generated U(1) gauge field. The gauge fluctuations are not confining due to coupling to holons, but yield an attractive interaction between spinons and holons leading to a bound state with electron quantum numbers. The renormalisation effects due to gauge fluctuations give rise to non--Fermi liquid behaviour for the composite electron.This formalism provides a new interpretation of the spin gap in underdoped superconductors (due to short-ranged AF order) and predicts the minimal gap for the physical electron is proportional to the square root of the doping concentration.

cond-mat.str-el↗
arXiv1998

Fermi Surface Evolution, Pseudo Gap and Stagger Gauge Field Fluctuation in Underdoped Cuprates

In the context of t-J model we show that in underdoped regime,beside the usual long wave length gauge field fluctuation, an additional low energy fluctuation, staggered gauge field fluctuation plays a crucial role in the evolution of Fermi surface(FS) as well as the line shape of spectral function for the cuprates. By including the staggered gauge field fluctuation we calculate the spectral function of the electrons by RPA(random phase approximation). The line shape of the spectral function near $(π,0)$ is very broad in underdoped case and is quite sharp in overdoped case. For the spectral function near $(0.5π,0.5π)$, the quasiparticle peaks are always very sharp in both underdoped and overdoped case. The temperature dependence of the spectral function is also discussed in our present calculation. These results fit well with the recent ARPES experiments. We also calculate the FS crossover from a small four segment like FS to a large continuous FS. The reason of such kind of FS crossover is ascribed to the staggered gauge field fluctuation which is strong in underdoped regime and becomes much weaker in overdoped regime. The pseudo gap extracted from the ARPES data can be also interpreted by the calculation.

cond-mat.str-el↗
arXiv1997

Effective Mass of Composite Fermions and Fermionic Chern-Simons Theory in Temporal Gauge

The definitions of the effective mass of the composite fermion are discussed for the half-filled Landau level problem. In a recent work, Shankar and Murthy show a finite effective mass of the composite fermion by a canonical transformation while the perturbative calculation gives the logarithmic divergence of the effective mass at the Fermi surface. We will emphasize that the different definition of the effective mass has the different physical processes. The finite one could be defined for any momentum of the composite fermion while the divergence only appears at the Fermi surface. We work with the standard Halperin-Lee-Read model but in the temporal gauge. The advantage of this gauge to be employed is that the finite effective mass could be calculated in the Hartree-Fock approximation. Furthermore, it is precisely equal to the result that Shankar and Murthy obtained which is well-fit with the numerical calculation from the ground state energy analysis and a semi-classical estimation. However, if we consider the random phase approximation, one sees that the divergence of the effective mass of the quasiparticle at the Fermi surface emerges again no matter that we work on the temporal or Coulomb gauges. We develop an effective theory where the finite effective mass serves as a `bare' effective mass and show that the same divergence of the RPA effective mass. On the other hand, the correct behavior of the response functions in the small band mass limit could be seen clearly in the temporal gauge since there is a self-interaction among the magnetoplasmons.

cond-mat.mes-hall↗
arXiv1997

$E\otimes e$ Jahn-Teller Effect in $C_{70}^{3-}$ Systems

The electron-phonon interaction in $C_{70}$ anions is studied by making use of a lattice relaxation approach. We find there exists a Jahn-Teller effect in $C_{70}^{3-}$ system, due to an extra electron being doped to the double degenerate $E_{1}^{''}$ state. As a result of this effect, the original $D_{5h}$ symmetry of the ground state becomes unstable, which causes distortion of the lattice configuration. The only symmetry maintained in the final state of the relaxation is the $x-y$ plane reflection symmetry. We further find that besides the Jahn-Teller active $A_{1}^{'}, A_{2}^{'}, E_{2}^{'}$ modes, the non-Jahn-Teller active $E_{1}^{'}$ vibrations also contribute to the relaxation process, which come from the nonlinear effect and are two or three orders smaller than those of the Jahn-Teller active modes. We suggest that the $C_{70}^{3-}$ molecule is a promising Berry Phase candidate in this effective $E \otimes e$ Jahn-Teller system.

cond-mat.mtrl-sci↗
arXiv1997

Spin Precession and Time-Reversal Symmetry Breaking in Quantum Transport of Electrons Through Mesoscopic Rings

We consider the motion of electrons through a mesoscopic ring in the presence of spin-orbit interaction, Zeeman coupling, and magnetic flux. The coupling between the spin and the orbital degrees of freedom results in the geometric and the dynamical phases associated with a cyclic evolution of spin state. Using a non-adiabatic Aharonov-Anandan phase approach, we obtain the exact solution of the system and identify the geometric and the dynamical phases for the energy eigenstates. Spin precession of electrons encircling the ring can lead to various interference phenomena such as oscillating persistent current and conductance. We investigate the transport properties of the ring connected to current leads to explore the roles of the time-reversal symmetry and its breaking therein with the spin degree of freedom being fully taken into account. We derive an exact expression for the transmission probability through the ring. We point out that the time-reversal symmetry breaking due to Zeeman coupling can totally invalidate the picture that spin precession results in effective, spin-dependent Aharonov-Bohm flux for interfering electrons. Actually, such a picture is only valid in the Aharonov-Casher effect induced by spin-orbit interaction only. Unfortunately, this point has not been realized in prior works on the transmission probability in the presence of both SO interaction and Zeeman coupling. We carry out numerical computation to illustrate the joint effects of spin-orbit interaction, Zeeman coupling and magnetic flux. By examining the resonant tunneling of electrons in the weak coupling limit, we establish a connection between the observable time-reversal symmetry breaking effects manifested by the persistent current and by the transmission probability. For a ring formed by two-dimensional electron gas, we

cond-mat.mes-hall↗
arXiv1997

Coil Formation in Multishell Carbon Nanotubes: Competition between Curvature Elasticity and Interlayer Adhesion

To study the shape formation process of carbon nanotubes, a string equation describing the possible existing shapes of the axis-curve of multishell carbon tubes (MCTs) is obtained in the continuum limit by minimizing the shape energy, that is the difference between the MCT energy and the energy of the carbonaceous mesophase (CM). It is shown that there exists a threshold relation of the outmost and inmost radii, that gives a parameter regime in which a straight MCT will be bent or twisted. Among the deformed shapes, the regular coiled MCTs are shown being one of the solutions of the string equation. In particular,the optimal ratio of pitch $p$ and radius $r_0$ for such a coil is found to be equal to $2π$, which is in good agreement with recent observation of coil formation in MCTs by Zhang et al.

cond-mat↗
arXiv1996

Non-Fermi liquid behavior in an extended Anderson model

An extended Anderson model, including screening channels (non-hybridizing, but interacting with the local orbit), is studied within the Anderson-Yuval approach, originally devised for the single-channel Kondo problem. By comparing the perturbation expansions of this model and a generalized resonant level model, the spin-spin correlation functions are calculated which show non-Fermi liquid exponent depending on the strength of the scattering potential. The relevance of this result to experiments in some heavy fermion systems is briefly discussed.

cond-mat↗
arXiv1996

Persistent Current From the Competition Between Zeeman Coupling and Spin-Orbit Interaction

Applying the non-adiabatic Aharonov-Anandan phase approach to a mesoscopic ring with non-interacting many electrons in the presence of the spin-orbit interaction, Zeeman coupling and magnetic flux, we show that the time-reversal symmetry breaking due to Zeeman coupling is intrinsically different from that due to magnetic flux. We find that the direction of the persistent currents induced by the Zeeman coupling changes periodically with the particle number, while the magnetic flux determines the direction of the induced currents by its sign alone.

cond-mat↗
arXiv1996

Dimensional reduction of U(1)\times SU(2) Chern-Simons bosonization: application to the t-J model

We perform a dimensional reduction of the $U(1)\times SU(2)$ Chern--Simons bosonization and apply it to the $t-J$ model, relevant for high $T_c$ superconductors. This procedure yields a decomposition of the electron field into a product of two ``semionic" fields, i.e. fields obeying abelian braid statistics with statistics parameter $θ={1\over 4}$, one carrying the charge and the other the spin degrees of freedom. A mean field theory is then shown to reproduce correctly the large distance behaviour of the correlation functions of the 1D $t-J$ model at $t>>J$. This result shows that to capture the essential physical properties of the model one needs a specific ``semionic" form of spin--charge separation.

hep-th↗
arXiv1995

Universality class of non-Fermi liquid behavior in mixed valence systems

A generalized Anderson single-impurity model with off-site Coulomb interactions is derived from the extended three-band Hubbard model, originally proposed to describe the physics of the copper-oxides. Using the abelian bosonization technique and canonical transformations, an effective Hamiltonian is derived in the strong coupling limit, which is essentially analogous to the Toulouse limit of the ordinary Kondo problem. In this limit, the effective Hamiltonian can be exactly solved, with a mixed valence quantum critical point separating two different Fermi liquid phases, {\it i.e.} the Kondo phase and the empty orbital phase. In the mixed valence quantum critical regime, the local moment is only partially quenched and X-ray edge singularities are generated. Around the quantum critical point, a new type of non-Fermi liquid behavior is predicted with an extra specific heat $C_{imp}\sim T^{1/4}$ and a singular spin-susceptibility $χ_{imp}\sim T^{-3/4}$. At the same time, the effective Hamiltonian under single occupancy is transformed into a resonant-level model, from which the correct Kondo physical properties (specific heat, spin susceptibility, and an enhanced Wilson ratio) are easily rederived. Finally, a brief discussion is given to relate these theoretical results to observations in $UPd_xCu_{5-x}$ ($x=1,1.5$) alloys, which show single-impurity critical behavior consistent with our predictions.

cond-mat↗
arXiv1995

Edge states in Open Antiferromagnetic Heisenberg Chains

In this letter we report our results in investigating edge effects of open antiferromagnetic Heisenberg spin chains with spin magnitudes $S=1/2, 1,3/2,2$ using the density-matrix renormalization group (DMRG) method initiated by White. For integer spin chains, we find that edge states with spin magnitude $S_{edge}=S/2$ exist, in agreement with Valence-Bond-Solid model picture. For half-integer spin chains, we find that no edge states exist for $S=1/2$ spin chain, but edge state exists in $S=3/2$ spin chain with $S_{edge}=1/2$, in agreement with previous conjecture by Ng. Strong finite size effects associated with spin dimmerization in half-integer spin chains will also be discussed.

cond-mat↗
arXiv1994

Theory of Second Order Optical Processes from A Luttinger Liquid

We develop a theory for the total optical secondary emission from a 1D interacting electron system modelled as a Tomonaga-Luttinger liquid. We separate the emission into two parts which may originate in {\em hot luminescence} (HL) and {\em Raman Scattering} (RS) respectively when we neglect the {\em interference} effect. We find a peak around $Δω= v_f |q|$ in the RS part which does not come from a structure factor peak. In general the total emission cannot be separated into HL and RS. However at resonance, and taking into account the $k$ dependence of the optical matrix element, a part of the RS is proportional to the structure factor $S(q_1-q_2, ω_1-ω_2)$.

cond-mat↗
arXiv1994

Quantum fluctuations of $D_{5d}$ polarons on $C_{60}$ molecules

The dynamic Jahn-Teller splitting of the six equivalent $D_{5d}$ polarons due to quantum fluctuations is studied in the framework of the Bogoliubov-de Gennes formalism. The tunneling induced level splittings are determined to be $^2 T_{1u} \bigoplus ^2 T_{2u}$ and $^1 A_g \bigoplus ^1 H_g$ for $C_{60}^{1-}$ and $C_{60}^{2-}$, respectively, which should give rise to observable effects in experiments.

cond-mat↗
arXiv1993

The Constraint for the Lowest Landau Level and the Chern-Simons Field Theory Approach for the Fractional Quantum Hall Effect: Infinite and Finite Systems

We build the constraint that all electrons are in the lowest Landau level into the Chern-Simons field theory approach for the fractional quantum Hall system. We show that the constraint can be transmitted from one hierarchical state to the next. As a result, we derive in generic the equations of the fractionally charged vortices ( quasi-particles ) for arbitrary hierarchy filling. For a finite system, we show that the action for each hierarchical state can be divided into two parts: the surface part provides the action for the edge excitations while the remaining bulk part is exactly the action for the next hierarchical states. In particular, we not only show that the surface action for the edge excitations would be decoupled from the bulk at each hierarchy filling, but also derive the explicit expressions analytically for the drift velocities of the hierarchical edge excitations.

cond-mat↗