Quantum electrodynamics at finite temperature

We present a systematic examination of finite temperature effects in quantum electrodynamics at one loop order. We calculate mass, charge, and wavefunction renormalization, demonstrate the running of the coupling constant at high temperatures, and study the renormalized vertex function and the energy momentum tensor. The confusion in the literature concerning the finite temperature corrections to the electron's magnetic moment is resolved. We also present the finite temperature effects in scalar electrodynamics. Throughout we stress the need to provide a well-defined method to observe a given quantity when interpreting the results of a calculation, and we suggest new techniques which incorporate the novel features of finite temperature theories.     

Photon sector of quantum electrodynamics at high temperature in two spatial dimensions

The photon sector of quantum electrodynamics (QED) in two spatial dimensions is analyzed at high temperature to all orders of perturbation theory. Imaginary-time formalism is used. The photon self-energy and propagator at finite temperature with vanishing frequency is calculated to the second order of perturbation theory. Based upon the latter, an improved perturbation theory which incorporates Debye screening is formulated. By virtue of the latter and gauge invariance, infrared finitness holds. The temperature dependence of any contribution to the connected Green's functions in the improved perturbation theory is analyzed systematically. At very high temperature, the photon sector becomes equivalent to a very massive scalar boson field plus a massless electromagnetic field and both become decoupled: all connected Green's functions containing, at least, one closed fermion loop with four or more vertices are shown to tend to zero.     

Exact finite temperature quantum statistics of single atom electrodynamics in a cavity of arbitrary Q

We report a complete and exact analysis of the steady-state physical properties characterising a single radiating atom in a cavity of arbitrary Q. Solutions are in terms of multitime correlation functions and no approximation is made in terms of Q, temperature, or strength of field-atom coupling g. Results are import also to fundamental concepts of statistical mechanics such as adiabatic following and Markov approximations.     

Many-phonon processes and the interaction of spinless quasiparticles with lattice vibrations at finite temperature

Using diagram techniques, we present an exact system of equations for the total mass operator of spinless quasiparticles interacting with phonons at finite temperature. The mass operator is then represented in the form of a branching fraction and the n-th arbitrary term is determined exactly. Using an exactly solvable example, it is shown that this representation of the mass operator can be used to take into account many-phonon processes generating the spectrum of the quasiparticles in an efficient way.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 93–98, October, 1986.     

Remarks on supersymmetric quantum mechanics at finite temperature

We discuss the finite temperature behavior of supersymmetric quantum mechanics. We give some general results on energy eigenvalues and eigenstates and discuss their implications on susy breaking. For some models we derive exact solutions. In addition we remark on peculiarities occurring if the formalism of thermo field dynamics is used in supersymmetric theories.     

Time evolution for quantum systems at finite temperature

This paper investigates a new formalism to describe real time evolution of quantum systems at finite temperature. A time correlation function among subsystems will be derived which allows for a probabilistic interpretation. Our derivation is non-perturbative and fully quantized. Various numerical methods used to compute the needed path integrals in complex time were tested and their effectiveness was compared. For checking the formalism we used the harmonic oscillator where the numerical results could be compared with exact solutions. Interesting results were also obtained for a system that presents tunneling. A ring of coupled oscillators was treated in order to try to check self-consistency in the thermodynamic limit. The short time distribution seems to propagate causally in the relativistic case. Our formalism can be extended easily to field theories where it remains to be seen if relevant models will be computable.     

Proper energy of an electron in a topologically massive (2+1) quantum electrodynamics system at finite temperature and density

The one-loop approximation is used to calculate the effects of finite temperature and nonzero chemical potential on the electron energy shift in a (2+1)-quantum electrodynamic system containing a Churn-Simon term. The induced electron mass is derived with a massless (2+1)-quantum electrodynamic system together with the exchange correction to the thermodynamic potential for a completely degenerate electron gas. It is shown that in the last case, incorporating the Churn-Simon term leads to loss of the gap in the direction law.Moscow State Institute for Electronics and Mathematics (Technical University). Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 61–65, May, 1995.     

Isolating vacuum amplitudes in quantum field calculations at finite temperature

In calculating Feynman diagrams at finite temperature, it is sometimes convenient to isolate subdiagrams which do not depend explicitly on the temperature. We show that, in the imaginary time formalism, such a separation can be achieved easily by exploiting a simple method, due to Gaudin, to perform the sum over the Matsubara frequencies. In order to manipulate freely contributions which may be individually singular, a regularization has to be introduced. We show that, in some cases, it is possible to choose this regularization in such a way that the isolated subdiagrams can be identified with analytical continuations of vacuum n-point functions. However, at least with the regularization used in this paper, this simple analytical structure does not hold for arbitrary diagrams, as revealed by counter-examples. As an aside illustration of Gaudin's method, we use it to prove the main part of a recent conjecture for a relation, in the imaginary time formalism, between the expressions of a Feynman diagram at zero and finite temperature.     

Sensitive chemical compass and quantum criticality at finite temperature

We investigate the influence of a flip operation of the central spin on the quantum criticality of a radical pair system by employing the spin echo and its product yield. It is found that with echo control on the central spin, the critical behavior can be described by the product yield at very high temperatures. Moreover, we also study the short and long time behavior of the spin echo, and show that the decay factor of the short time evolution scales linearly. The long time evolution shows different statistics for varying chain lengths, temperature and external parameters of the Hamiltonian.     

A quantum field theory of lattice dynamics at finite temperature

A quantum field theoretical treatment of three-dimensional cubic crystals at finite temperature is presented from the view-point of the spontaneous breakdown of the spatial translational invariance using thermo field theory. The effective interaction Hamiltonian is constructed by taking into account the dynamical map of the molecular density operator which is obtained from the Ward-Takahashi relations. The acoustic phonons are expected to be the excitation of particle-hole pairs. The conventional secular equation for the lattice vibrations is obtained by neglecting some quantum effects in the Bethe-Salpeter equation for the molecular density fluctuations. The phonon spectra, the phonon propagators and the dynamical map of the molecular density operator are calculated at finite temperature.     

Deconfining and chiral phase transitions in quantum chromodynamics at finite temperature

We give further arguments to support the claim of Svetitsky and Yaffe that the finite-temperature transition in 4-dimensional SU(N) gauge theories is in the universality class of 3-dimensional Z_N spin models. We show that this implies a smoothing out of the transition when quarks are added to the system as long as N ≠ 3. For N = 3 the pure gauge transition is expected to be first order and will be smoothed by quarks only if the quark contribution to the internal energy is larger than the latent heat of transition.     

Phonon effect on two coupled quantum dots at finite temperature

The quantum oscillations of population in an asymmetric double quantum dots system coupled to a phonon bath are investigated theoretically. It is shown how the environmental temperature has effect on the system.     

在有限温度下运用弱测量保护量子纠缠

根据单量子比特的映射, 提出一个在有限温度下运用弱测量保护两量子比特纠缠的方案. 在两个可通过局域幺正变化实现的等价的初始纠缠进入广义振幅阻尼信道前后, 对其分别进行弱测量, 并对四个弱测量的参数做全面的优化, 获取最大共生纠缠C_r 和弱测量参数m, n的解析表达式, 然后再进一步研究弱测量参数与信道参数的关系. 发现这种基于弱测量的纠缠保护方法在某些情况下可以有效地提高纠缠, 甚至可以避免纠缠的突然死亡. 当信道参数r一定时, 对不同的参数p,初始态|ψ>纠缠达到最大值时对应的弱测量参数m的取值一样, 共生纠缠关于p=0.5对称, 而初始态|φ>对应的参数m的取值不同; 在参数p一定、参数r不同、初始态|ψ>或|φ>的情况下, 当纠缠度取最大值时, 弱测量参数m的取值不变, 且随着r的增加, 纠缠度减少. 通过对信道参数的分析, 发现可以选择合适的信道参数和初始态来获得较大的纠缠.     

Single-particle distribution function of a quantum dot system at finite temperature

In the present paper, we shall rigorously re-establish the result of the single-particle function of a quantum dot system at finite temperature. Unlike the proof given in our previous work (Phys. Rev. B 74 195414 (2006)), we take a different approach, which does not exploit the explicit expression of the Gibbs distribution function. Instead, we only assume that the statistical distribution function of the quantum dot system is thermodynamically stable. As a result, we are able to show clearly that the electronic structure in the quantum dot system is completely determined by its thermodynamic stability. Furthermore, the weaker requirements on the statistical distribution function also make it possible to apply the same method to the quantum dot systems in non-equilibrium states.     

Electronic properties of a large quantum dot at a finite temperature

The physical properties of a two-dimensional parabolic quantum dot composed of large number of interacting electrons are numerically determined by the Thomas–Fermi (TF) method at a finite temperature. Analytical solutions are given for zero temperature for comparative purposes. The exact solution of the TF equation is obtained for the non-interacting system at finite temperatures. The effect of the number of particles and temperature on the properties are investigated both for interacting and non-interacting cases. The results indicate that the effect of e–e interaction on the density profile shows different temperature dependencies above and below a certain temperature T_c.