631 research outputs found
Excitonic phase transition in the extended three-dimensional Falicov-Kimball model
We study the excitonic phase transition in a system of the conduction band
electrons and valence band holes described by the three-dimensional (3D)
extended Falicov-Kimball (EFKM) model with the tunable Coulomb interaction
between both species. By lowering the temperature, the electron-hole system may
become unstable with respect to the formation of the excitons, i.e,
electron-hole pairs at temperature , exhibiting a gap in
the particle excitation spectrum. To this end we implement the functional
integral formulation of the EFKM, where the Coulomb interaction term is
expressed in terms of U(1) phase variables conjugate to the local particle
number, providing a useful representation of strongly correlated system. The
effective action formalism allows us to formulate a problem in the phase-only
action in the form of the quantum rotor model and to obtain analytical formula
for the critical lines and other quantities of physical interest like charge
gap, chemical potential and the correlation length.Comment: 27 pages, 15 figures (in the arXive version), 37 pages and 15 figures
(in the published version
Effective pairing interaction in the two-dimensional Hubbard model within a spin rotationally invariant approach
We implement the rotationally-invariant formulation of the two-dimensional
Hubbard model, with nearest-neighbors hopping , which allows for the
analytical study of the system in the low-energy limit. Both U(1) and SU(2)
gauge transformations are used to factorize the charge and spin contributions
to the original electron operator in terms of the corresponding gauge fields.
The Hubbard Coulomb energy term is then expressed in terms of quantum phase
variables conjugate to the local charge and variable spin-quantization axis,
providing a useful representation of strongly correlated systems. It is shown
that these gauge fields play a similar role as phonons in the BCS theory: they
act as the "glue" for fermion pairing. By tracing out gauge degrees of freedom,
the form of paired states is established and the strength of the pairing
potential is determined. It is found that the attractive pairing potential in
the effective low-energy fermionic action is non-zero in a rather narrow range
of .Comment: 12 pages, 1 figur
The conduction pathway of potassium channels is water free under physiological conditions.
Ion conduction through potassium channels is a fundamental process of life. On the basis of crystallographic data, it was originally proposed that potassium ions and water molecules are transported through the selectivity filter in an alternating arrangement, suggesting a "water-mediated" knock-on mechanism. Later on, this view was challenged by results from molecular dynamics simulations that revealed a "direct" knock-on mechanism where ions are in direct contact. Using solid-state nuclear magnetic resonance techniques tailored to characterize the interaction between water molecules and the ion channel, we show here that the selectivity filter of a potassium channel is free of water under physiological conditions. Our results are fully consistent with the direct knock-on mechanism of ion conduction but contradict the previously proposed water-mediated knock-on mechanism
Temperature-doping phase diagram of layered superconductors
The superconducting properties of a layered system are analyzed for the cases
of zero- and non-zero angular momentum of the pairs. The effective
thermodynamic potential for the quasi-2D XY-model for the gradients of the
phase of the order parameter is derived from the microscopic superconducting
Hamiltonian. The dependence of the superconducting critical temperature T_c on
doping, or carrier density, is studied at different values of coupling and
inter-layer hopping. It is shown that the critical temperature T_c of the
layered system can be lower than the critical temperature of the
two-dimensional Berezinskii-Kosterlitz-Thouless transition T_BKT at some values
of the model parameters, contrary to the case when the parameters of the
XY-model do not depend on the microscopic Hamiltonian parameters.Comment: To be published in Phys. Rev.
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The structure of a potassium-selective ion channel reveals a hydrophobic gate regulating ion permeation
Protein dynamics are essential to function. One example of this is the various gating mechanisms within ion channels, which are transmembrane proteins that act as gateways into the cell. Typical ion channels switch between an open and closed state via a conformational transition which is often triggered by an external stimulus, such as ligand binding or pH and voltage differences. The atomic resolution structure of a potassium-selective ion channel named NaK2K has allowed us to observe that a hydrophobic residue at the bottom of the selectivity filter, Phe92, appears in dual conformations. One of the two conformations of Phe92 restricts the diameter of the exit pore around the selectivity filter, limiting ion flow through the channel, while the other conformation of Phe92 provides a larger-diameter exit pore from the selectivity filter. Thus, it can be concluded that Phe92 acts as a hydrophobic gate, regulating the flow of ions through the selectivity filter
Analysis of time-profiles with in-beam PET monitoring in charged particle therapy
Background: Treatment verification with PET imaging in charged particle
therapy is conventionally done by comparing measurements of spatial
distributions with Monte Carlo (MC) predictions. However, decay curves can
provide additional independent information about the treatment and the
irradiated tissue. Most studies performed so far focus on long time intervals.
Here we investigate the reliability of MC predictions of space and time (decay
rate) profiles shortly after irradiation, and we show how the decay rates can
give an indication about the elements of which the phantom is made up.
Methods and Materials: Various phantoms were irradiated in clinical and
near-clinical conditions at the Cyclotron Centre of the Bronowice proton
therapy centre. PET data were acquired with a planar 16x16 cm PET system.
MC simulations of particle interactions and photon propagation in the phantoms
were performed using the FLUKA code. The analysis included a comparison between
experimental data and MC simulations of space and time profiles, as well as a
fitting procedure to obtain the various isotope contributions in the phantoms.
Results and conclusions: There was a good agreement between data and MC
predictions in 1-dimensional space and decay rate distributions. The fractions
of C, O and C that were obtained by fitting the decay
rates with multiple simple exponentials generally agreed well with the MC
expectations. We found a small excess of C in data compared to what was
predicted in MC, which was clear especially in the PE phantom.Comment: 9 pages, 5 figures, 1 table. Proceedings of the 20th International
Workshop on Radiation Imaging Detectors (iWorid2018), 24-28 June 2018,
Sundsvall, Swede
Theory of a spherical quantum rotors model: low--temperature regime and finite-size scaling
The quantum rotors model can be regarded as an effective model for the
low-temperature behavior of the quantum Heisenberg antiferromagnets. Here, we
consider a -dimensional model in the spherical approximation confined to a
general geometry of the form (
-linear space size and -temporal size) and subjected to periodic
boundary conditions. Due to the remarkable opportunity it offers for rigorous
study of finite-size effects at arbitrary dimensionality this model may play
the same role in quantum critical phenomena as the popular Berlin-Kac spherical
model in classical critical phenomena. Close to the zero-temperature quantum
critical point, the ideas of finite-size scaling are utilized to the fullest
extent for studying the critical behavior of the model. For different
dimensions and a detailed analysis, in terms of the
special functions of classical mathematics, for the susceptibility and the
equation of state is given. Particular attention is paid to the two-dimensional
case.Comment: 33pages, revtex+epsf, 3ps figures included submitted to PR
Time reparametrization group and the long time behaviour in quantum glassy systems
We study the long time dynamics of a quantum version of the
Sherrington-Kirkpatrick model. Time reparametrizations of the dynamical
equations have a parallel with renormalization group transformations, and
within this language the long time behaviour of this model is controlled by a
reparametrization group (RG) fixed point of the classical dynamics. The
irrelevance of the quantum terms in the dynamical equations in the aging regime
explains the classical nature of the violation of the fluctuation-dissipation
theorem.Comment: 4 page
Quantum phase transitions and thermodynamic properties in highly anisotropic magnets
The systems exhibiting quantum phase transitions (QPT) are investigated
within the Ising model in the transverse field and Heisenberg model with
easy-plane single-site anisotropy. Near QPT a correspondence between parameters
of these models and of quantum phi^4 model is established. A scaling analysis
is performed for the ground-state properties. The influence of the external
longitudinal magnetic field on the ground-state properties is investigated, and
the corresponding magnetic susceptibility is calculated. Finite-temperature
properties are considered with the use of the scaling analysis for the
effective classical model proposed by Sachdev. Analytical results for the
ordering temperature and temperature dependences of the magnetization and
energy gap are obtained in the case of a small ground-state moment. The forms
of dependences of observable quantities on the bare splitting (or magnetic
field) and renormalized splitting turn out to be different. A comparison with
numerical calculations and experimental data on systems demonstrating magnetic
and structural transitions (e.g., into singlet state) is performed.Comment: 46 pages, RevTeX, 6 figure
Interaction-induced Bose Metal in 2D
We show here that the regularization of the conductivity resulting from the
bosonic interactions on the `insulating' (quantum disordered) side of an
insulator-superconductor transition in 2D gives rise to a metal with a finite
conductivity, , as temperature tends to zero. The Bose
metal is stable to weak disorder and hence represents a concrete example of an
interaction-induced metallic phase. Phenomenological inclusion of dissipation
reinstates the anticipated insulating behaviour in the quantum-disordered
regime. Hence, we conclude that the traditionally-studied
insulator-superconductor transition, which is driven solely by quantum
fluctuations, corresponds to a superconductor-metal transition. The possible
relationship to experiments on superconducting thin films in which a
low-temperature metallic phase has been observed is discussed.Comment: A figure has been added and the physics has been clarified. To appear
in PR
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