arXiv subjects

A. Furusaki

Publications and source records attributed to A. Furusaki.

47 records · Page 3Linked to original sources

arXiv1998

Correlation amplitude for S=1/2 XXZ spin chain in the critical region

The density-matrix renormalization-group technique is used to calculate the spin correlation functions and of the one-dimensional S=1/2 XXZ model in the gapless regime. The numerical results for open chains of 200 spins are analyzed by comparing them with correlation functions calculated from a low-energy field theory. This gives precise estimates of the amplitudes of the correlation functions in the thermodynamic limit. The exact amplitude recently conjectured by Lukyanov and Zamolodchikov and the logarithmic correction in the Heisenberg model are confirmed numerically.

cond-mat.stat-mech↗

arXiv1998

Kondo effect in XXZ spin chains

The Kondo effect in a one-dimensional spin-1/2 XXZ model in the gapless XY regime (-1<Delta<=1) is studied both analytically and numerically. In our model an impurity spin (S=1/2) is coupled to a single spin in the XXZ spin chain. Perturbative renormalization-group (RG) analysis is performed for various limiting cases to deduce low-energy fixed points. It is shown that in the ground state the impurity spin is screened by forming a singlet with a spin in the host XXZ chain. In the antiferromagnetic side (0<Delta<=1) the host chain is cut into two semi-infinite chains by the singlet. In the ferromagnetic side (-1<Delta<0), on the other hand, the host XXZ chain remains as a single chain through "healing" of a weakened bond in the low-energy (long-distance) limit. The density matrix renormalization group method is used to study the size scaling of finite-size energy gaps and the power-law decay of correlation functions in the ground state. The numerical results are in good agreement with the predictions of the RG analysis. Low-temperature behaviors of specific heat and susceptibility are also discussed.

cond-mat.str-el↗

arXiv1997

Resonant tunneling through a quantum dot weakly coupled to quantum wires or quantum Hall edge states

Resonant tunneling through a quantum dot weakly coupled to Tomonaga-Luttinger liquids is discussed. The linear conductance due to sequential tunneling is calculated by solving a master equation for temperatures below and above the average level spacing in the dot. When the parameter g characterizing the Tomonaga-Luttinger liquid is smaller than 1/2, the resonant tunneling process is incoherent down to zero temperature. At low temperature T the height and width of the conductance peaks in the Coulomb blockade oscillations are proportional to T^{(1/g)-2} and T, respectively. The contribution from tunneling via a virtual intermediate state (cotunneling) is also included. The resulting conductance formula can be applied for the resonant tunneling between edge states of fractional quantum Hall liquids with filling factor nu=1/(2m+1)=g.

cond-mat.mes-hall↗

arXiv1997

Local Perturbation in a Tomonaga-Luttinger Liquid at g=1/2

The orthogonality catastrophe in a Tomonaga-Luttinger liquid with an impurity is reexamined for the case when the interaction parameter or the dimensionless conductance is g=1/2. By transforming bosons back to fermions, the Hamiltonian is reduced to a quadratic form, which allows for explicit calculation of the overlap integral and the local density of states at the defect site. The exponent of the orthogonality catastrophe due to a backward scattering center is found to be 1/8, in agreement with previous studies using different approaches. The time-dependence of the core-hole Green's function is computed numerically, which shows a clear crossover from a non-universal short-time behavior to a universal long-time behavior. The local density of states vanishes linearly in the low-energy limit at g=1/2.

cond-mat.str-el↗

arXiv1996

The Low-Energy Fixed Points of Random Quantum Spin Chains

The one-dimensional isotropic quantum Heisenberg spin systems with random couplings and random spin sizes are investigated using a real-space renormalization group scheme. It is demonstrated that these systems belong to a universality class of disordered spin systems, characterized by weakly coupled large effective spins. In this large-spin phase the uniform magnetic susceptibility diverges as 1/T with a non-universal Curie constant at low temperatures T, while the specific heat vanishes as T^delta |ln T| for T->0. For broad range of initial distributions of couplings and spin sizes the distribution functions approach a single fixed-point form, where delta \approx 0.44. For some singular initial distributions, however, fixed-point distributions have non-universal values of delta, suggesting that there is a line of fixed points.

cond-mat.stat-mech↗

arXiv1996

Non-magnetic Impurities in Spin Gap Systems

The effect of non-magnetic impurities is discussed for both the two-leg Heisenberg ladder system and other spin liquids with excitation gap. It is shown that the random depletion of spins introduces a random Berry phase term to the non-linear sigma model. The classical nature of the antiferromagnetic correlation is enhanced by the topological decoherence, and the staggered susceptibility shows more singular behavior at low temperatures than the uniform antiferromagnetic Heisenberg chain.

cond-mat↗

arXiv1996

Low-Temperature Properties of the Randomly Depleted Heisenberg Ladder

Low-temperature thermodynamic properties of the spin-1/2 Heisenberg ladder system with non-magnetic impurities are discussed using an effective Hamiltonian for the impurity-induced spins in the background of a spin liquid with a gap. It is shown that the uniform susceptibility shows Curie-like behavior in high-, intermediate-, and low-temperature regimes, with three different Curie constants. The ratio of specific heat to temperature, C/T, is divergent in the low-temperature limit, reflecting a singular distribution of the effective couplings among the impurity spins.

cond-mat↗

arXiv1995

Theory of strong inelastic co-tunneling

We develop a theory of the conductance of a quantum dot connected to two leads by single-mode quantum point contacts. If the contacts are in the regime of perfect transmission, the conductance shows no Coulomb blockade oscillations as a function of the gate voltage. In the presence of small reflection in both contacts, the conductance develops small Coulomb blockade oscillations. As the temperature of the system is lowered, the amplitude of the oscillations grows, and eventually sharp periodic peaks in conductance are formed. Away from the centers of the peaks the conductance vanishes at low temperatures as $T^2$, in agreement with the theory of inelastic co-tunneling developed for the weak-tunneling case. Conductance near the center of a peak can be studied using an analogy with the multichannel Kondo problem. In the case of symmetric barriers, the peak conductance at $T\to 0$ is of the order of $e^2/\hbar$. In the asymmetric case, the peak conductance vanishes linearly in temperature.

cond-mat↗

arXiv1995

Random Exchange Quantum Heisenberg Chains

The one-dimensional quantum Heisenberg model with random $\pm J$ bonds is studied for $S=\frac{1}{2}$ and $S=1$. The specific heat and the zero-field susceptibility are calculated by using high-temperature series expansions and quantum transfer matrix method. The susceptibility shows a Curie-like temperature dependence at low temperatures as well as at high temperatures. The numerical results for the specific heat suggest that there are anomalously many low-lying excitations. The qualitative nature of these excitations is discussed based on the exact diagonalization of finite size systems.

cond-mat↗

arXiv1995

Random Quantum Spin Chains: A Real-Space Renormalization Group Study

Quantum Heisenberg spin chains with random couplings and spin sizes are studied using a real-space renormalization group technique. These systems belong to a new universality class of disordered quantum spin systems realized in {\it e.g.} ${\rm Sr}_3{\rm CuPt}_{1-x}{\rm Ir}_x {\rm O}_6$. The low-energy fixed point is characterized by the formation of weakly coupled large effective spins. At low temperature $T$ the entropy obeys a power law $T^δ$ ($δ\approx0.44$), and the susceptibility follows a Curie-like behavior.

cond-mat↗

arXiv1995

Coulomb blockade oscillations of conductance in the regime of strong tunneling

We study the transport through a quantum dot coupled to two leads by single-mode point contacts. The linear conductance is calculated analytically as a function of a gate voltage and temperature T in the case when transmission coefficients of the contacts are close to unity. As a function of the gate voltage, the conductance shows Coulomb blockade oscillations. At low temperatures, the off-resonance conductance vanishes as T^2, in agreement with the theory of inelastic co-tunneling. Near a resonance, the low-energy physics is governed by a multi-channel Kondo fixed point.

cond-mat↗