Gauge field thermodynamics for the SU(2) Yang-Mills system

After reviewing the euclidean formulation of the thermodynamics for quantum spin systems, we develop the corresponding formalism for SU(N) gauge fields on the lattice. The results are then evaluated for the SU(2) system, using Monte Carlo simulation on lattices of (space × temperature) size 10³ × 2,3,4,5. At high temperature, the system exhibits Stefan-Boltzmann behaviour, with three gluonic colour degrees of freedom. At T_c ≈ 43Λ_L (215 MeV), the transition to “hadronic” behaviour occurs, signalled by a sharp peak in the specific heat. From the behaviour below the deconfinement transition (T<T_c), we obtain m_G ≈ 200Λ_L (1000 MeV) for the mass of the lowest gluonium state (glueball).     

Quantum Phase Transition and Thermal Entanglement in the Ising Model with Dzyaloshinskii-Moriya Interaction in an Inhomogeneous Magnetic Field

We investigate the quantum phase transition (QPT) and thermal entanglement in the two-qubit Ising model in an inhomogeneous magnetic field, and the DM interaction between the two lattices is considered. The results show the QPT highly relates to the magnetic intensity B and DM parameter D, by controlling the DM interaction D and external magnetic B we can change the positions of QPT points and the level spacing. Moreover, the QPT is closely related to thermal entanglement, when the QPT happens the ground state always changes between entanglement state and disentanglement state. The thermal entanglement highly depends on the system’s temperature T, DM intensity D and external magnetic field B. When T is lower, the entanglement can exhibit a platform-like region. By modulating the parameters D and B, the entanglement can be controlled and the entanglement switch can be realized.     

Zero-temperature ordering in the two-dimensional quantum XY model

We have calculated the probability distribution for the staggered magnetization at T=0 for the 2D antiferromagnetic quantum XY model on finite lattices. For the ground state, the distribution shows evidence of isotropic magnetization ordering on the xy-plane. Based on data on seven lattices up to 26 sites, the extrapolated value of the staggered magnetization is 0.448±0.003 in the thermodynamic limit.     

Critical behaviour of SU(n) quantum chains and topological non-linear σ-models

The critical behaviour of SU(n) quantum “spin” chains, Wess-Zumino-Witten σ-models and grassmanian σ-models at topological angle θ = π (of possible relevance to the quantum Hall effect) is reexamined. It is argued that an additional Z_n symmetry is generally necessary to stabilize the massless phase. This symmetry is not present for the σ-models for n > 2 and is only present for certain representations of “spin” chains.     

Quantum Coherence and Correlation in Spin Models with Dzyaloshinskii-Moriya Interaction

We study quantum coherence and quantum correlation for detecting quantum phase transition (QPT) by means of quantum renormalization group (QRG) in various spin chain models with Dzyaloshinskii-Moriya (DM) interaction, including XXZ model with DM interaction, Ising model with DM interaction and XY model with DM interaction. It is found that through enough QRG iterations, l ₁ norm quantum coherence and one-norm geometric quantum discord can effectively characterize QPT. We also discuss the effect of DM interaction and anisotropy on quantum coherence and quantum correlation.     

Quantum Correlation Properties Between Two Spatially Separated Atoms in Free Space Reinforced by Incoherent Pumping

Quantum correlation dynamics between two identical and spatially separated atoms in free space is investigated by the use of concurrence C and quantum discord (QD). The behaviors of QD differs in many unexpected ways from the entanglement in this system. Firstly, it shows the situations which the concurrence and QD can behave very differently with a “sudden birth” phenomenon of the former but not of the latter, and QD is only oscillating decays with time and the interqubit distance. We also verify the cases which QD is always greater than the concurrence and the region where the concurrence is vanished but with nonzero values for QD. Meanwhile an unexpected situation which the concurrence is greater than QD under the initial state |eg〉 is analyzed. It is revealed that the quantum correlation based only on QD is expected to be more robust than entanglement which is not suitable for all the initial states under the decoherence environment. Then, by introducing the incoherent pumping, we also study the different properties of the steady-state entanglement and QD about this atomic subsystem. It is shown that the incoherent pumping can overcome the decay of the atoms and the influences about the interqubit distance r ₁₂/λ on the steady-state correlation can make the decay of the concurrence obviously quicker than QD, the life of the steady-state QD is evidently larger than the steady-state entanglement.     

A quantum spherical model of spin glass

A quantum spherical model of spin glass is solved. The solution exhibits the usual spin glass phase transition, but with-out any unphysical features at low temperatures. In particular, the entropy is always positive and vanishes like T³ as T → 0.     

Incoherent tunneling at weak bias: Strong quantum damping

Using a generalized version of Langer's “imaginary part of the free energy” method involving an “extended” bounce and doing a fully “dynamical” evaluation of the weak bias anomalous fluctuation mode, the thermal dependence of the decay rate Γ = A exp(-B) is studied for a metastable quartic potential model in the strong ohmic quantum damping regime in order to extend previous work to a considerably more realistic range of parameters. In particular, the earlier restrictions 12 B exp(-ħω₀/2k_BT)→0 and 2πk_BT/ħω₀→0 are removed. As a consequence there appear - inter alia - T² -corrections in B similar to those occuring for strongly biased metastable systems.     

Photoreflectance study on the photovoltaic effect in InAs/GaAs quantum dot solar cell

To investigate the effect of quantum dot (QD) layers on the photovoltaic process of InAs/GaAs QD solar cell (QDSC), QD layers were embedded in conventional GaAs p-n junction SC (GaAs SC) structures. The photoreflectance (PR) was examined at different temperatures (T) and excitation light intensities (I_ex) to investigate the photovoltaic effects through observation of the Franz-Keldysh oscillations (FKOs) in the PR spectra. The evaluated the p-n junction electric fields (F_pn) of the InAs QDSC was different from that of the GaAs SC. Moreover, InAs QDSC show that the different photovoltaic behaviors compared with GaAs SC by varying I_ex and T. From these considerations, we suggest that the different photovoltaic behaviors are caused by the effect of the additional photo-carrier generation in InAs QD layers resulting in enhancement of the field screening effect in F_pn.     

Observation of subgap resistive oscillations in doubly connected SNS systems with the suppressed proximity effect

Phase-sensitive magnetoresistive oscillations were observed at temperatures T<4 K in hybrid quasi ballistic doubly connected SNS structures with single-crystal normal spacers of macroscopic sizes in all dimensions (L=100–500 μm) and elastic electron mean free path on the same scale l _el～100 μm. The oscillations observed for the distances corresponding to the indicated L values between the NS interfaces are evidence that the phase-interruption length in pure metal is macroscopic at T<4 K. The oscillation parameters related to the geometry of structures and spacer sizes indicate the quantum nature of the oscillations. These are shown to reflect the behavior of conductance in the NS interfaces with the dissipative coherent transport of “ subgap” quasiparticles with energies ε?Δ, T (Δ is the gap energy).     

Propagating energy modes in the heisenberg chain: Just plain magnons

We show that the oscillations in the energy density observed in classical simulations on ferromagnetic chains are present in the antiferromagnet as well, and are not due to hydrodynamic “second magnon” modes, but arise from a singularity in the non interacting two spin wave density of states.Quantum occupation number effects change qualitatively the nature of the oscillations, producing a perfectly well defined mode at the spin wave frequency as T→0.     

Optical and electronic properties of quantum dots with magnetic impurities

The article discusses some of the recent results on semiconductor quantum dots with magnetic impurities. A single Mn impurity incorporated in a quantum dot strongly changes the optical response of a quantum-dot system. A character of Mn-carrier interaction is very different for II-VI and III-V quantum dots (QDs). In the II-VI QDs, a Mn impurity influences mostly the spin-structure of an exciton. In the III-V dots, a spatial localization of hole by a Mn impurity can be very important, and ultimately yields a totally different spin structure. A Mn-doped QD with a variable number of mobile carriers represents an artificial magnetic atom. Due to the Mn-carrier interaction, the order of filling of electronic shells in the magnetic QDs can be very different to the case of the real atoms. The “periodic” table of the artificial magnetic atoms can be realized in voltage-tunable transistor structures. For the electron numbers corresponding to the regime of Hund's rule, the magnetic Mn-carrier coupling is especially strong and the magnetic-polaron states are very robust. Magnetic QD molecules are also very different to the real molecules. QD molecules can demonstrate spontaneous breaking of symmetry and phase transitions. Single QDs and QD molecules can be viewed as voltage-tunable nanoscale memory cells where information is stored in the form of robust magnetic-polaron states. To cite this article: A.O. Govorov, C. R. Physique 9 (2008).     

Impurity and exciton effects on the nonlinear optical properties of a disc-like quantum dot under a magnetic field

A detailed investigation of the nonlinear optical properties of the (D⁺, X) complex in a disc-like quantum dot (QD) with the parabolic confinement, under applied magnetic field, has been carried by using the perturbation method and the compact density-matrix approach. The linear and nonlinear optical absorption coefficients between the ground (L = 0) and the first excited state (L = 1) have been examined based on the computed energies and wave functions. The competition between the confinement and correlation effects on the one hand, and the magnetic field effects on the other hand, is also discussed. The results show that the confinement strength of QDs and the intensity of the illumination have drastic effects on the nonlinear optical properties. In addition, we note that the absorption coefficients of an exciton in QDs depend strongly on the impurity but weakly on the magnetic field. Furthermore, the light and heavy hole excitons should be taken into account when we study the optical properties of an exciton in a disc-like QD.     

Large-scale anisotropies of the cosmic background radiation in generalized inflationary cosmologies

We calculate the dipole and quadropole moments, (ΔT/T)_l l=1, 2, of the cosmic background radiation anisotropy due to scalar and tensor waves with power-law spectra, produced by quantum processes during a “generalized” inflationary era of the early universe. The observational bounds on (ΔT/T)_l are used to give constraints on the parameters of the cosmological models.     

Almost-adiabatic collisions of quantum systems

Expressions are derived for corrections resulting from a small variation of the “slow” internal degrees of freedom during an almost-adiabatic quantum collision. First-order non-adiabatic contributions to the scattering wave function and scattering T-matrix element are derived making use of (a) Wronskian surface terms, and (b) converging factors. Advantages of method (a) are indicated. Modifications of the expressions necessary to include a possible projectile intrinsic spin are described. A one-dimensional model illustrating the range of validity of the adiabatic approximation is presented.     

Dynamically induced Kondo effect in double quantum dots

A new mechanism of resonance Kondo tunneling via a composite quantum dot (QD) is proposed. It is shown that, owing to the hidden dynamic spin symmetry, the Kondo effect can be induced by a finite voltage eV applied to the contacts at an even number N of electrons in a QD with zero spin in the ground state. As an example, a double QD is considered in a parallel geometry with N=2, which possesses the SO(4) type symmetry characteristic of a singlet-triplet pair. In this system, the Kondo peak of conductance appears at an eV value compensating for the exchange splitting.     

Thermal tripartite quantum correlations: quantum discord and entanglement perspectives

We investigate thermal tripartite quantum correlations for a spin star network and for a new extended version of it. In a spin star network, three peripheral spins interact with the central spin identically while in extended spin star network, three peripheral spins interact with two central spatially separated spins in the same way. We exploit the method of [C.C. Rulli, M.S. Sarandy, Phys. Rev. A 84, 042109 (2011)] to evaluate the tripartite quantum discord (TQD) and the method of [M. Li, S. Fei, Z. Wang, Rep. Math. Phys 65, 289 (2010)] called as lower bound of tripartite concurrence (LBTC) to evaluate the tripartite entanglement (TE) of the the peripheral parties in both systems. It is found that thermal TQD is much more robust than thermal TE as a function of temperature T. Also, the peripheral parties of the extended spin star network, in comparison with those of the spin star one, can exhibit higher values of TQD at T > 0. This, indeed, motivates us to realise improved quantum information and quantum computation tasks at finite temperatures.     

Exact quantum critical points and phase separation instabilities in Betts Hubbard nanoclusters

Spontaneous phase separation instabilities with the formation of various types of charge and spin pairing (pseudo)gaps in U>0 Hubbard model including the next nearest neighbor coupling are calculated with the emphasis on the two-dimensional (square) lattices generated by 8- and 10-site Betts unit cells. The exact theory yields insights into the nature of quantum critical points, continuous transitions, dramatic phase separation instabilities and electron condensation in spatially inhomogeneous systems. The picture of coupled antiparallel (singlet) spins and paired charged holes suggests full Bose condensation and coherent pairing in real space at zero temperature of electrons complied with the Bose-Einstein statistics. Separate pairing of charge and spin degrees at distinct condensation temperatures offers a new route to superconductivity different from the BCS scenario. The conditions for spin liquid behavior coexisting with unsaturated and saturated Nagaoka ferromagnetism due to spin-charge separation are established. The phase separation critical points and classical criticalities found at zero and finite temperatures resemble a number of inhomogeneous, coherent and incoherent nanoscale phases seen near optimally doped high-T_c cuprates, pnictides and CMR nanomaterials.     

Global quantum discord and matrix product density operators

In a previous study, we have proposed a procedure to study global quantum discord in 1D chains whose ground states are described by matrix product states [Z.-Y. Sun et al., Ann. Phys. 359, 115 (2015)]. In this paper, we show that with a very simple generalization, the procedure can be used to investigate quantum mixed states described by matrix product density operators, such as quantum chains at finite temperatures and 1D subchains in high-dimensional lattices. As an example, we study the global discord in the ground state of a 2D transverse-field Ising lattice, and pay our attention to the scaling behavior of global discord in 1D sub-chains of the lattice. We find that, for any strength of the magnetic field, global discord always shows a linear scaling behavior as the increase of the length of the sub-chains. In addition, global discord and the so-called “discord density” can be used to indicate the quantum phase transition in the model. Furthermore, based upon our numerical results, we make some reliable predictions about the scaling of global discord defined on the n × n sub-squares in the lattice.     

Coupling of quantum logics

A quantum logic is a couple (L, M), whereL is a logic andM is a quite full set of states onL. A tensor product in the category of quantum logics is defined and a comparison with the definition of free orthodistributive product of orthomodular σ lattices is given. Several physically important cases are treated.