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Register a new userThermodynamics of a Spin-1/2 Chain Coupled to Einstein Phonons
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A high order series expansion is employed to study the thermodynamical properties of a S=1/2 chain coupled to dispersionless phonons. The results are obtained without truncating the phonon subspace since the series expansion is performed formally in the overall exchange coupling J. The results are used to investigate various parameter regimes, e.g. the adiabatic and antiadiabatic limit as well as the intermediate regime which is difficult to investigate by other methods. We find that dynamic phonon effects become manifest when more than one thermodynamic quantity is analyzed.
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We study the thermodynamics of an XYZ Heisenberg chain with Dzyaloshinskii-Moriya interaction, which describes the low-energy behaviors of a one-dimensional spin-orbit-coupled bosonic model in the deep insulating region. The entropy and the specific heat are calculated numerically by the quasi-exact transfer-matrix renormalization group. In particular, in the limit $U^prime/Urightarrowinfty$, our model is exactly solvable and thus serves as a benchmark for our numerical method. From our data, we find that for $U^prime/U>1$ a quantum phase transition between an (anti)ferromagnetic phase and a Tomonaga-Luttinger liquid phase occurs at a finite $theta$, while for $U^prime/U<1$ a transition between a ferromagnetic phase and a paramagnetic phase happens at $theta=0$. A refined ground-state phase diagram is then deduced from their low-temperature behaviors. Our findings provide an alternative way to detect those distinguishable phases experimentally.
Field-dependent specific heat and neutron scattering measurements were used to explore the antiferromagnetic S=1/2 chain compound CuCl2 * 2((CD3)2SO). At zero field the system acquires magnetic long-range order below TN=0.93K with an ordered moment of 0.44muB. An external field along the b-axis strengthens the zero-field magnetic order, while fields along the a- and c-axes lead to a collapse of the exchange stabilized order at mu0 Hc=6T and mu0 Hc=3.5T, respectively (for T=0.65K) and the formation of an energy gap in the excitation spectrum. We relate the field-induced gap to the presence of a staggered g-tensor and Dzyaloshinskii-Moriya interactions, which lead to effective staggered fields for magnetic fields applied along the a- and c-axes. Competition between anisotropy, inter-chain interactions and staggered fields leads to a succession of three phases as a function of field applied along the c-axis. For fields greater than mu0 Hc, we find a magnetic structure that reflects the symmetry of the staggered fields. The critical exponent, beta, of the temperature driven phase transitions are indistinguishable from those of the three-dimensional Heisenberg magnet, while measurements for transitions driven by quantum fluctuations produce larger values of beta.
Inelastic neutron scattering was used to measure the magnetic field dependence of spin excitations in the antiferromagnetic S=1/2 chain CuCl_2 2(dimethylsulfoxide) (CDC) in the presence of uniform and staggered fields. Dispersive bound states emerge from a zero-field two-spinon continuum with different finite energy minima at wave numbers q=pi and q_i approx pi (1-2<S_z>). The ratios of the field dependent excitation energies are in excellent agreement with predictions for breather and soliton solutions to the quantum sine-Gordon model, the proposed low-energy theory for S=1/2 chains in a staggered field. The data are also consistent with the predicted soliton and n=1,2 breather polarizations and scattering cross sections.
We employ matrix-product state techniques to numerically study the zero-temperature spin transport in a finite spin-1/2 XXZ chain coupled to fermionic leads with a spin bias voltage. Current-voltage characteristics are calculated for parameters corresponding to the gapless XY phase and the gapped Neel phase. In both cases, the low-bias spin current is strongly suppressed unless the parameters of the model are fine-tuned. For the XY phase, this corresponds to a conducting fixed point where the conductance agrees with the Luttinger-liquid prediction. In the Neel phase, fine-tuning the parameters similarly leads to an unsuppressed spin current with a linear current-voltage characteristic at low bias voltages. However, with increasing the bias voltage, there occurs a sharp crossover to a region where a current-voltage characteristic is no longer linear and the smaller differential conductance is observed. We furthermore show that the parameters maximizing the spin current minimize the Friedel oscillations at the interface, in agreement with the previous analyses of the charge current for inhomogeneous Hubbard and spinless fermion chains.
Single crystals of a metal organic complex ce{(C5H12N)CuBr3} (ce{C5H12N} = piperidinium, pipH for short) have been synthesized and the structure was determined by single-crystal X-ray diffraction. ce{(pipH)CuBr3} crystallizes in the monoclinic group $C$2/$c$. Edging-sharing ce{CuBr5} units link to form zigzag chains along the $c$ axis and the neighboring Cu(II) ions with spin-1/2 are bridged by bi-bromide ions. Magnetic susceptibility data down to 1.8 K can be well fitted by the Bonner-Fisher formula for antiferromagnetic spin-1/2 chain, giving the intrachain magnetic coupling constant $J$ $sim$ 17 K. At zero field, ce{(pipH)CuBr3} shows three-dimensional (3D) order below $T_N$ = 1.68 K. Calculated by the mean-field theory, the interchain coupling constant $J$ = 0.65 K is obtained and the ordered magnetic moment $m_0$ is about 0.20 $mu_B$. This value of $m_0$ makes ce{(pipH)CuBr3} a rare compound suitable to study the dimensional crossover problem in magnetism, since both 3D order and one-dimensional (1D) quantum fluctuations are prominent. In addition, specific heat measurements reveal two successive magnetic transitions with lowering temperature when external field $H geq$ 3 T is applied along the $a$ axis. The $H$ - $T$ phase diagram of ce{(pipH)CuBr3} is roughly constructed. The interplay between exchange interactions, dimensionality, Zeeman energy and possible Dzyaloshinkii-Moriya interaction should be the driving force for the multiple phase transitions.
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