Coherence vs. decoherence in (some) problems of condensed matter physics

We present an 'overview' of coherence-to-decoherence transition in certain selected problems of condensed matter physics. Our treatment is based on a subsystem-plus-environment approach. All the examples chosen in this paper have one thing in common - the environmental degrees of freedom are taken to be bosonic and their spectral density of excitations is assumed to be 'ohmic'. The examples are drawn from a variety of phenomena in condensed matter physics involving, for instance, quantum diffusion of hydrogen in metals, Landau diamagnetism and c-axis transport in high Tc superconductors

The myth about Einstein

In common perception, Einstein comes out as a strong mathematical physicist. This is however a myth. The {dy1905} Einstein was close to real life phenomena. This article presents how he used simple mathematics to understand experiments, especially on Brownian Motion and Photoelectric Effect, employing the underlying concept of thermodynamic fluctuations

Quantum Treatment of the Anderson-Hasegawa Model -- Effects of Superexchange and Polarons

We revisit the Anderson-Hasegawa double-exchange model and critically examine its exact solution when the core spins are treated quantum mechanically.We show that the quantum effects, in the presence of an additional superexchange interaction between the core spins, yield a term, the significance of which has been hitherto ignored. The quantum considerations further lead to new results when polaronic effects, believed to be ubiquitous in manganites due to electron-phonon coupling, are included. The consequence of these results for the magnetic phase diagrams and the thermal heat capacity is also carefully analysed.Comment: 18 pages, Revtex, 7 postscript figure

Dissipative diamagnetism with anomalous coupling and third law

In this work, low temperature thermodynamic behaviour in the context of dissipative diamagnetism with anomalous coupling is analyzed. We find that finite dissipation substitutes the zero-coupling result of exponential decay of entropy by a power law behaviour at low temperature. For Ohmic bath, entropy vanishes linearly with temperature, $T$, in conformity with Nernst's theorem. It is also shown that entropy decays faster in the presence of anomalous coupling than that of the usual coordinate-coordinate coupling. It is observed that velocity-velocity coupling is the most advantageous coupling scheme to ensure the third law of thermodynamics. It is also revealed that different thermodynamic functions are independent of magnetic field at very low temperature for various coupling schemes discussed in this work.Comment: 18 pages, 3 figure

Microscopic analysis of relaxation behavior in nonlinear optical conductivity of graphene

We present here a general formulation for the interband dynamical optical conductivity in the nonlinear regime of graphene in the presence of a quantum bath comprising phonons and electrons. Our main focus is the relaxation behavior of the quantum solid of graphene perturbed by an oscillatory electric field. Considering the optical range of the frequency and a considerable amount of the amplitude of the field, one can observe a nonlinear response by formulating a quantum master equation of the density operator associated with the Hamiltonian encapsulated in the form of a spin-Boson model of dissipative quantum statistical mechanics. Mapping the valence and conduction states as the eigenstates of the Pauli spin operators and utilizing the rotating wave approximation to omit off-resonant terms, one can solve the rate equation for the mean population of the conduction and valence states and the mixing matrix elements between them. Our results reveal the nonlinear steady-state regime's population inversion and interband coherence. It is characterized by a single dimensionless parameter that is directly proportional to the incident field strength and inversely proportional to the optical frequency. Our method is also capable of calculating the nonlinear interband optical conductivity of doped and gapped graphene at finite temperatures. The effects of different bath spectra for phonons and electrons are examined in detail. Although our general formulation can address a variety of nonequilibrium response of the two-band system, it also facilitates a connection with phenomenological modeling of nonlinear optical conductivity

Dephasing of a Qubit due to Quantum and Classical Noise

The qubit (or a system of two quantum dots) has become a standard paradigm for studying quantum information processes. Our focus is Decoherence due to interaction of the qubit with its environment, leading to noise. We consider quantum noise generated by a dissipative quantum bath. A detailed comparative study with the results for a classical noise source such as generated by a telegraph process, enables us to set limits on the applicability of this process vis a vis its quantum counterpart, as well as lend handle on the parameters that can be tuned for analyzing decoherence. Both Ohmic and non-Ohmic dissipations are treated and appropriate limits are analyzed for facilitating comparison with the telegraph process.Comment: 12 pages, 8 figure