Crossover from a pseudogap state to a superconducting state

This paper deduces that the particular electronic structure of cuprate superconductors confines Cooper pairs to be first formed in the antinodal region which is far from the Fermi surface,and these pairs are incoherent and result in the pseudogap state.With the change of doping or temperature,some pairs are formed in the nodal region which locates the Fermi surface,and these pairs are coherent and lead to superconductivity.Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface.It also shows that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure,and non-s wave symmetry gap favours the high-temperature superconductivity.Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.     

Lyapunov exponents from quantum dynamics

We extract classical Lyapunov exponents from the time dependence of quantum mechanical expectation values. Classical chaos is revealed as a quantum transient with a liftetime ～? ln ?. Our strategy is shown to work for the example of a periodically kicked top.     

Crossover from non-Fermi liquid to Fermi liquid behavior close to a quantum critical point: A brief review

The nesting of the Fermi surfaces of an electron and a hole pocket separated by a nesting vector Q and the interaction between electrons gives rise to itinerant antiferromagnetism. The order can gradually be suppressed by mismatching the nesting and a quantum critical point is obtained as the Néel temperature tends to zero. We review our results on the specific heat, the quasi-particle linewidth, the electrical resistivity, the amplitudes of de Haas-van Alphen oscillations and the dynamical spin susceptibility.     

The excitation energy spectrum of liquid He II based on theU-matrix theory

Summary In this paper, we apply theU-matrix theory to derive an explicit expression for the excitation energy spectrum of liquid⁴He. Using a model for the effective chemical potential for⁴He, we are able to produce an excitation spectrum which is very close to the observed one. The inverse of the effective mass,i.e. 1/m ^*, is obtained as a function of momentumk. The ratio between the effective mass and the mass of helium atom atk=2.0 ?⁻¹,i.e. near the roton depth, is found to bem ^*(k=2.0 ?⁻¹)/m _He=0.18298, while the ratio atk=1.0 ?⁻¹ ism ^*(k=1.0 ?⁻¹)/m _He=−0.17103. The theoretical result of the excitation spectrum is consistent with observational data.

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Riassunto In questo lavoro si applica la teoria della matriceU per dedurre un'espressione esplicita per lo spettro d'energia di eccitazione dell'⁴He liquido. Usando un modello per il potenziale chimico efficace per l'⁴He si è in grado di produrre uno spettro di eccitazione molto simile a quello osservato. L'inverso della massa efficace, cioè 1/m ^*, è ottenuto in funzione del momentok. Si trova che il rapporto tra massa efficace e la massa dell'atomo di elio ak=2.0 ?⁻¹, cioè vicino alla profondità del rotore, èm ^* (k=2.0 ?⁻¹)/m _He= =0.18298, mentre quello ak=1.0 ?⁻¹ èm ^* (k=1.0 ?⁻¹)/m _he=−0.17103. Il risultato teorico dello spettro di eccitazione è coerente con i dati sperimentali.

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Резюме В этой статье мы применяем теориюU-матрицы для вывода явного выражения для энергетического спектра возбуждений Зидкого⁴He. Используя модель для эффективного химического потенциала для⁴He, мы воспроизводим спектр возбуждений который оказывается очень близким к зкспериментально наблюдаемому. Вычисляется обратная величина эффективной массы, т.е. 1/m ^*, как функция импульсаk. Получено отношение эффективной массы к массе атома гелия приk=2.0 ?⁻¹, которое составляетm ^* (k=2.0 ?⁻¹)/m _He=0.18298, тогда как это отношение приk=1.0 ?⁻¹ равноm ^* (k=1.0 ?⁻¹)/m _He=−0.17103. Теоретический результат для спектра возбужений согласуется с имеющимися экспериментальными данными.

     

Crossover from quantum to classical transport

Understanding the crossover from quantum to classical transport has become of fundamental importance not only for technological applications due to the creation of sub-10-nm transistors – an important building block of our modern life – but also for elucidating the role played by quantum mechanics in the evolutionary fitness of biological complexes. This article provides a basic introduction into the nature of charge and energy transport in the quantum and classical regimes. It discusses the characteristic transport properties in both limits and demonstrates how they can be connected through the loss of quantum mechanical coherence. The salient features of the crossover physics are identified, and their importance in opening new transport regimes and in understanding efficient and robust energy transport in biological complexes are demonstrated.     

Quantum state transfer from light to molecules via coherent two-color photo-association in an atomic Bose-Einstein condensate

By using a quantized input light, we theoretically revisit the coherent two-color photo-association process in an atomic Bose-Einstein condensate. Under the single-mode approximations, we show two interesting regimes of the light transmission and the molecular generation. The quantum state transfer from light to molecules is exhibited, without or with the depletion of trapped atoms.     

Quantizing gravity and spacetime: Where do we stand ?

We review five possible solutions to the riddle posed by Quantum Gravity: (1) Gravity should stay as a classical theory (L. Rosenfeld); (2) Quantum Gravity requires a formalism which will take the human mind (or the intelligent observer) into account, resolving at the same time the riddle of the collapse of the wave function/state vector in Quantum Mechanics in general (Penrose); (3) Perturbative Quantization; (4) Hamiltonian Quantization (Dirac, Ashtekar); (5) String Theory. We also discuss the quantization of spacetime.     

Brouillage thermique d'un gaz cohérent de fermions

It is generally assumed that a condensate of paired fermions at equilibrium is characterized by a macroscopic wavefunction with a well-defined, immutable phase. In reality, all systems have a finite size and are prepared at non-zero temperature; the condensate has then a finite coherence time, even when the system is isolated in its evolution and the particle number N is fixed. The loss of phase memory is due to interactions of the condensate with the excited modes that constitute a dephasing environment. This fundamental effect, crucial for applications using the condensate of pairs' macroscopic coherence, was scarcely studied. We link the coherence time to the condensate phase dynamics, and we show with a microscopic theory that the time derivative of the condensate phase operator ${\hat{\theta }}_0$ is proportional to a chemical potential operator that we construct including both the pair-breaking and pair-motion excitation branches. In a single realization of energy E, ${\hat{\theta }}_0$ evolves at long times as $-2{\mu }_{\mathrm{mc}}(E)t/ħ$, where ${\mu }_{\mathrm{mc}}(E)$ is the microcanonical chemical potential; energy fluctuations from one realization to the other then lead to a ballistic spreading of the phase and to a Gaussian decay of the temporal coherence function with a characteristic time $\propto N^{1/2}$. In the absence of energy fluctuations, the coherence time scales as N due to the diffusive motion of ${\hat{\theta }}_0$. We propose a method to measure the coherence time with ultracold atoms, which we predict to be tens of milliseconds for the canonical ensemble unitary Fermi gas.     

Connections between quantum chromodynamics and condensed matter physics

Features of QCD can be seen qualitatively in certain condensed matter systems. Recently some of the analyses that originated in condensed matter physics have found applications in QCD. Using examples we discuss some of the connections between the two fields and show how progress can be made by exploiting this connection. Some of the challenges that remain in the two fields are quite similar. We argue that recent algorithmic developments call for optimism in both fields.     

Critical parameters of hard-core Yukawa fluids within the structural theory

A purely statistical mechanical approach is proposed to account for the liquid–vapor critical point based on the mean density approximation (MDA) of the direct correlation function. The application to hard-core Yukawa (HCY) fluids facilitates the use of the series mean spherical approximation (SMSA). The location of the critical parameters for HCY fluid with variable intermolecular range is accurately calculated. Good agreement is observed with computer simulation results and with the inverse temperature expansion (ITE) predictions. The influence of the potential range on the critical parameters is demonstrated and the universality of the critical compressibility ratio is discussed. The behavior of the isochoric and isobaric heat capacities along the equilibrium line and the near vicinity of the critical point is discussed in details.     

Working group report: Quark gluon plasma

The 10th Workshop on High Energy Physics Phenomenology (WHEPP-10) was held at the Institute of Mathematical Sciences, Chennai during January 2–13, 2008. One of our working grops (WG) is QCD and QGP. The discussions of QGP WG include matter at high density, lattice QCD, charmonium states in QGP, viscous hydrodynamics and jet quenching, colour factor in heavy ion collisions and RHIC results on photons, dileptons and heavy quark. There were two plenary talks and several working group talks with intense discussions regarding the future activities that are going to be persued.      

Plane-chain coupling in YBa2Cu3O7: temperature dependence of the penetration depth

We have studied the penetration depth for a model of YBa₂Cu₃O₇ involving pairing, both in the CuO₂ planes and in the CuO chains. In this model pairing in the planes is due to an attractive interaction, while Coulomb repulsion induces in the chains an order parameter with opposite sign. Due to the anti-crossing produced by hybridization between planes and chains, one obtains a d-wave like order parameter which changes sign on a single sheet of the Fermi surface and has nodes in the gap. We find that our model accounts quite well for the anisotropy of the penetration depth and for the absolute values. We reproduce fairly well the whole temperature dependence for both the a- and the b-directions, including the linear dependence at low temperature. We use a set of parameters which are all quite reasonable physically. Our results for the c-direction are also satisfactory, although the situation is less clear both experimentally and theoretically.     

Simple condensation of composite bosons in a number conserving approach to many fermion-systems

We recently proposed an exact bosonization procedure which generates a Hamiltonian of composite bosons interacting among themselves and with fermionic quasiparticles. The interaction among composites whose mixing is allowed by symmetries is strong, but a simple condensate cannot have a significant mixing with other composites. We determine the conditions of decoupling, study their effects and compare the results with the Random Phase Approximation and the BCS theory.     

Fermion Corrections to Mass of Scalar Glueball in QCD Sum Rule

Contributions of fermions to the mass of the scalar glueball 0^＋＋ are calculated at two-loop level in the framework of QCD sum rules. It slightly changes the coefficients in the operator product expansion （OPE） and shifts the mass of glueball to 1.72 ± 0.07 GeV.     

Superconductivity in the cuprates: Deduction of mechanism for d-wave pairing through analysis of ARPES

In the Eliashberg integral equations for d-wave superconductivity, two different functions (α²F)_n(ω, θ) and (α²F)_p,d(ω) determine, respectively, the “normal” self-energy and the “pairing” self-energy. ω is the frequency of fluctuations scattering the fermions whose momentum is near the Fermi-surface and makes an angle θ to a chosen axis. We present a quantitative analysis of the high-resolution laser based Angle Resolved Photoemission Spectroscopy (ARPES) data on a slightly under doped cuprate compound Bi2212 and use the Eliashberg equations to deduce the ω and θ dependence of (α²F)_n(ω, θ) for T just above T_c and below T_c. Besides its detailed ω dependence, we find the remarkable result that this function is nearly independent of θ between the (π; π)-direction and 25 degrees from it, except for the dependence of the cut-off energy on θ. Assuming that the same fluctuations determine both the normal and the pairing self-energy, we ask what theories give the function (α²F)_p,d(ω) required for the d-wave pairing instability at high temperatures as well as the deduced (α²F)_n(θ, ω). We show that the deduced (α²F)_n(θ, ω) can only be obtained from antiferromagnetic (AFM) fluctuations if their correlation length is smaller than a lattice constant. Using (α²F)_p,d(ω) consistent with such a correlation length and the symmetry of matrix-elements scattering fermions by AFM fluctuations, we calculate T_c and show that AFM fluctuations are excluded as the pairing mechanism for d-wave superconductivity in cuprates. We also consider the quantumcritical fluctuations derived microscopically as the fluctuations of the observed loop–current order discovered in the under-doped cuprates, and which lead to the marginal Fermi–liquid properties in the normal state. We show that their frequency dependence and the momentum dependence of their matrix-elements to scatter fermions are consistent with the θ and ω dependence of the deduced (α²F)_n(ω, θ). The pairing kernel (α²F)_p,d(ω) calculated using the experimental values in the Eliashberg equation gives d-wave instability at T_c comparable to the experiments.