Infinite randomness fixed point of the superconductor-metal quantum phase transition

We examine the influence of quenched disorder on the superconductor-metal transition, as described by a theory of overdamped Cooper pairs which repel each other. The self-consistent pairing eigenmodes of a quasi-one-dimensional wire are determined numerically. Our results support the recent proposal by Hoyos et al. [Phys. Rev. Lett. 99, 230601 (2007)10.1103/PhysRevLett.99.230601] that the transition is characterized by the same strong-disorder fixed point describing the onset of ferromagnetism in the random quantum Ising chain in a transverse field.     

Universal conductance of nanowires near the superconductor-metal quantum transition

We consider wires near a zero temperature transition between superconducting and metallic states. The critical theory obeys hyperscaling, which leads to a universal frequency, temperature, and length dependence of the conductance; quantum and thermal phase slips are contained within this critical theory. Normal, superconducting, and mixed (SN) leads on the wire determine distinct universality classes. For the SN case, wires near the critical point have a universal dc conductance which is independent of the length of the wire at low temperatures.     

Universal scaling of the conductivity at the superfluid-insulator phase transition

The scaling of the conductivity at the superfluid-insulator quantum phase transition in two dimensions is studied by numerical simulations of the Bose-Hubbard model. In contrast to previous studies, we focus on properties of this model in the experimentally relevant thermodynamic limit at finite temperature T. We find clear evidence for deviations from omega k scaling of the conductivity towards omega k/T scaling at low Matsubara frequencies omega k. By careful analytic continuation using Padé approximants we show that this behavior carries over to the real frequency axis where the conductivity scales with omega/T at small frequencies and low temperatures. We estimate the universal dc conductivity to be sigma* = 0.45(5)Q2/h, distinct from previous estimates in the T = 0, omega/T > 1 limit.     

Exploring symmetry breaking at the Dicke quantum phase transition

We study symmetry breaking at the Dicke quantum phase transition by coupling a motional degree of freedom of a Bose-Einstein condensate to the field of an optical cavity. Using an optical heterodyne detection scheme, we observe symmetry breaking in real time and distinguish the two superradiant phases. We explore the process of symmetry breaking in the presence of a small symmetry-breaking field and study its dependence on the rate at which the critical point is crossed. Coherent switching between the two ordered phases is demonstrated.     

Dynamical quantum phase transition in XY chains with the Dzyaloshinskii-Moriya and XZY-YZX three-site interactions

We study the dynamical quantum phase transitions (DQPTs) in the $XY$ chains with the Dzyaloshinskii-Moriya interaction and the $XZY$-$YZX$ type of three-site interaction after a sudden quench. Both the models can be mapped to the spinless free fermion models by the Jordan-Wigner and Bogoliubov transformations with the form $H=\sum_{k}ǎrepsilon_{k}(\eta^{†}_{k}\eta_{k}-\frac{1}{2})$, where the quasiparticle excitation spectra $ǎrepsilon_{k}$ may be smaller than 0 for some $k$ and are asymmetrical ($ǎrepsilon_{k}\neqǎrepsilon_{-k}$). It is found that the factors of Loschmidt echo equal 1 for some $k$ corresponding to the quasiparticle excitation spectra of the pre-quench Hamiltonian satisfying $ǎrepsilon_{k}\cdotǎrepsilon_{-k}<0$, when the quench is from the gapless phase. By considering the quench from different ground states, we obtain the conditions for the occurrence of DQPTs for the general $XY$ chains with gapless phase, and find that the DQPTs may not occur in the quench across the quantum phase transitions regardless of whether the quench is from the gapless phase to gapped phase or from the gapped phase to gapless phase. This is different from the DQPTs in the case of quench from the gapped phase to gapped phase, in which the DQPTs will always appear. Moreover, we analyze the different reasons for the absence of DQPTs in the quench from the gapless phase and the gapped phase. The conclusion can also be extended to the general quantum spin chains.     

Magnetic fluctuations at a field-induced quantum phase transition

We report an inelastic neutron-scattering study at the field-induced magnetic quantum phase transition of CeCu5.8Au0.2. The data can be described better by the spin-density-wave scenario than by a local quantum critical point, while the latter scenario was shown to be applicable to the zero-field concentration-tuned quantum phase transition in CeCu6-xAux for x=0.1. This constitutes direct microscopic evidence for a difference in the quantum fluctuation spectra at a magnetic quantum critical point driven by different tuning parameters.     

Dynamical scaling of the quantum Hall plateau transition

Using different experimental techniques, we examine the dynamical scaling of the quantum Hall plateau transition in a frequency range f=0.1-55 GHz. We present a scheme that allows for a simultaneous scaling analysis of these experiments and all other data in literature. We observe a universal scaling function with an exponent kappa=0.5+/-0.1, yielding a dynamical exponent z=0.9+/-0.2.     

Thermal conductivity at the amorphous-nanocrystalline phase transition in beech wood biocarbon

High-porosity samples of beech wood biocarbon (BE-C) were prepared by pyrolysis at carbonization temperatures T _carb = 650, 1300, and 1600°C, and their resistivity ρ and thermal conductivity κ were studied in the 5–300 and 80–300 K temperature intervals. The experimental results obtained were evaluated by invoking X-ray diffraction data and information on the temperature dependences ρ(T) and κ(T) for BE-C samples prepared at T _carb = 800, 1000, and 2400°C, which were collected by the authors earlier. An analysis of the κ(T _carb) behavior led to the conclusion that the samples under study undergo an amorphous-nanocrystalline phase transition in the interval 800°C < T _carb < 1000°C. Evaluation of the electronic component of the thermal conductivity revealed that the Lorentz number of the sample prepared at T _carb = 2400°C exceeds by far the classical Sommerfeld value, which is characteristic of metals and highly degenerate semiconductors.     

Fluctuation conductivity of thin films and nanowires near a parallel-field-tuned superconducting quantum phase transition

We calculate the fluctuation correction to the normal state conductivity in the vicinity of a quantum phase transition from a superconducting to a normal state, induced by applying a magnetic field parallel to a dirty thin film or a nanowire with thickness smaller than the superconducting coherence length. We find that at zero temperature, where the correction comes purely from quantum fluctuations, the positive "Aslamazov-Larkin" contribution, the negative "density of states" contribution, and the "Maki-Thompson" interference contribution are all of the same order and the total correction is negative. Further, we show that, based on how the quantum critical point is approached, there are three regimes that show different temperature and field dependencies which should be experimentally accessible.     

Quantum superconductor-metal transition in a proximity array

A theory of the zero-temperature superconductor-metal transition is developed for an array of superconductive islands (of size d) coupled via a disordered two-dimensional conductor with the dimensionless conductance g = Planck's over 2 pi/e(2)R(square)>1. At T = 0 the macroscopically superconductive state of the array with lattice spacing b>d is destroyed at g相似文献   

On the quantum phase transition of a quantum Hall liquid

It is shown that, in the absence of disorder, a quantum Hall liquid undergoes a second-order quantum phase transition as function of the applied magnetic field. The analysis is carried out in the framework of the Chem-Simons-Ginzburg-Landau theory which is shown to exhibit a nontrivial infrared stable fixed point. The corresponding critical exponents are found to be gaussian, and thus universal and independent of the filling fraction.     

Ideal soft mode-type quantum phase transition and phase coexistence at quantum critical point in 18O-exchanged SrTiO3

The quantum ferroelectric phase transition of 18O-exchanged SrTiO3 (x% exchanged SrTiO3 is abbreviated as STO18-x) was investigated by Raman scattering as a function of x. The result indicates the ideal soft mode-type quantum ferroelectric phase transition of STO18-x, where the 18O exchange enhances the softening of the soft mode by the suppression of quantum fluctuation. In the vicinity of the quantum critical point (x approximately xc=33%), the system results in the ferroelectric-paraelectric phase coexistence state, in clear contrast to the homogeneous ferroelectric phase in STO18-x, whose x is sufficiently larger than xc. Simultaneously, the softening of the soft mode becomes strongly rounded with the underdamped oscillation. The present result indicates that the sensitivity of the soft phonon vibration to the mass disorder is dramatically enhanced in the vicinity of the quantum critical point.     

Probing the plateau-insulator quantum phase transition in the quantum hall regime

We report quantum Hall experiments on the plateau-insulator transition in a low mobility In(0.53)Ga(0.47)As/InP heterostructure. The data for the longitudinal resistance rho(xx) follow an exponential law and we extract a critical exponent kappa = 0.55+/-0. 05 which is slightly different from the established value kappa = 0. 42+/-0.04 for the plateau transitions. Upon correction for inhomogeneity effects, which cause the critical conductance sigma(*)(xx) to depend marginally on temperature, our data indicate that the plateau-plateau and plateau-insulator transitions are in the same universality class.     

Nature of the quantum phase transition to a spin-nematic phase

It is shown that the quantum phase transition in metallic non-s-wave ferromagnets, or spin nematics, is generically of first order. This is due to a coupling of the order parameter to soft electronic modes that play a role analogous to that of the electromagnetic vector potential in a superconductor, which leads to a fluctuation-induced first-order transition. A generalized mean-field theory for the p-wave case is constructed that explicitly shows this effect. Tricritical wings are predicted to appear in the phase diagram in a spatially varying magnetic field, but not in a homogeneous one.     

Dynamics of a quantum phase transition

We present two approaches to the dynamics of a quench-induced phase transition in the quantum Ising model. One follows the standard treatment of thermodynamic second order phase transitions but applies it to the quantum phase transitions. The other approach is quantum, and uses Landau-Zener formula for transition probabilities in avoided level crossings. We show that predictions of the two approaches of how the density of defects scales with the quench rate are compatible, and discuss the ensuing insights into the dynamics of quantum phase transitions.