708 research outputs found
Quantum Ising model in a transverse random field: A density-matrix renormalization group analysis
The spin-1/2 quantum Ising chain in a transverse random magnetic field is
studied by means of the density-matrix renormalization group. The system
evolves from an ordered to a paramagnetic state as the amplitude of the random
field is increased. The dependence of the magnetization on a uniform magnetic
field in the z direction and the spontaneous magnetization as a function of the
amplitude of the transverse random magnetic field are determined. The behavior
of the spin-spin correlation function both above and at criticality is studied.
The scaling laws for magnetization and correlation functions are tested against
previous numerical and renormalization-group results.Comment: 5 pages with 7 figures inside them, proper format of authors' names
use
Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates
We consider the lightly doped cuprates YCaBaCuO
and LaSrCuO (with ,0.04), where the presence of a
fluctuating nematic state has often been proposed as a precursor of the stripe
(or, more generically, charge-density wave) phase, which sets in at higher
doping. We phenomenologically assume a quantum critical character for the
longitudinal and transverse nematic, and for the charge-ordering fluctuations,
and investigate the effects of these fluctuations in Raman spectra. We find
that the longitudinal nematic fluctuations peaked at zero transferred momentum
account well for the anomalous Raman absorption observed in these systems in
the channel, while the absence of such effect in the channel
may be due to the overall suppression of Raman response at low frequencies,
associated with the pseudogap. While in YCaBaCuO the
low-frequency lineshape is fully accounted by longitudinal nematic collective
modes alone, in LaSrCuO also charge-ordering modes with finite
characteristic wavevector are needed to reproduce the shoulders observed in the
Raman response. This different involvement of the nearly critical modes in the
two materials suggests a different evolution of the nematic state at very low
doping into the nearly charge-ordered state at higher doping.Comment: 12 pages with 10 figures, to appear in Phys. Rev. B 201
Extended paraconductivity regime in underdoped cuprates
We reconsider transport experiments in strongly anisotropic superconducting
cuprates and we find that universal Aslamazov-Larkin (AL) paraconductivity in
two dimensions is surprisingly robust even in the underdoped regime below the
pseudogap crossover temperature T^*. We also establish that the underlying
normal state resistivity in the pseudogap phase is (almost) linear in
temperature, with all the deviations being quantitatively accounted by AL
paraconductivity. The disappearence of paraconductivity is governed by the
disappearence of gaussian pair fluctuations at an energy scale related to T^*.Comment: 5 pages and 2 figure
Doping-dependent competition between superconductivity and polycrystalline charge density waves
From systematic analysis of the high pulsed magnetic field resistance data of
LaSrCuO thin films, we extract an experimental phase diagram
for several doping values ranging from the very underdoped to the very
overdoped regimes. Our analysis highlights a competition between charge density
waves and superconductivity which is ubiquitous between and
and produces the previously observed double step transition. When suppressed by
a strong magnetic field, superconductivity is resilient for two specific doping
ranges centered around respectively and and the
characteristic temperature for the onset of the competing charge density wave
phase is found to vanish above . At the two phases are found
to coexist exactly at zero magnetic field.Comment: 13 figures. Changes from previous version are in red. A few
clarifications and a discussion about the different materials were added
together with additional reference
Confinement of superconducting fluctuations due to emergent electronic inhomogeneities
The microscopic nature of an insulating state in the vicinity of a
superconducting state, in the presence of disorder, is a hotly debated
question. While the simplest scenario proposes that Coulomb interactions
destroy the Cooper pairs at the transition, leading to localization of single
electrons, an alternate possibility supported by experimental observations
suggests that Cooper pairs instead directly localize. The question of the
homogeneity, granularity, or possibly glassiness of the material on the verge
of this transition is intimately related to this fundamental issue. Here, by
combining macroscopic and nano-scale studies of superconducting ultrathin NbN
films, we reveal nanoscopic electronic inhomogeneities that emerge when the
film thickness is reduced. In addition, while thicker films display a purely
two-dimensional behaviour in the superconducting fluctuations, we demonstrate a
zero-dimensional regime for the thinner samples precisely on the scale of the
inhomogeneities. Such behavior is somehow intermediate between the Fermi and
Bose insulator paradigms and calls for further investigation to understand the
way Cooper pairs continuously evolve from a bound state of fermionic objects
into localized bosonic entities.Comment: 29 pages 9 figure
The positivity scale: Concurrent and factorial validity across late childhood and early adolescence
Despite the well-established protective functions of positivity (i.e., a dispositional selfevaluative tendency to view oneself, life, and future under a positive outlook) from middle adolescence to old age, its reliable assessment and contribution to a proper psychological functioning have received little attention during previous developmental phases. In this article, we aimed to evaluate the psychometric properties and construct validity of the eight-item Positivity Scale (P Scale; Caprara et al., 2012) during late childhood and early adolescence in a sample of British students (N = 742; 48% boys) from both primary (M age = 10.75, SD = 0.52) and secondary schools (M age = 13.38 years, SD = 0.94). First, results from confirmatory factor analysis (CFA) attested to the plausibility of the hypothesized 1-factor structure of the P Scale in a revised CFA model including the correlation between the residuals of two items similar in their wording. Next, we found evidence for strong (scalar) measurement invariance of the P Scale across late childhood and early adolescence as well as for its concurrent validity as indicated by expected relations of positivity to indicators of adjustment (i.e., prosocial behavior) and maladjustment (i.e., externalizing and internalizing problems). Overall, these findings support the concurrent and factorial validity of the P Scale as a short self-report instrument to measure children's tendency to view their experience from a positive stance. We discuss the implications of our results for improving the wording of the items composing P Scale as well as for understanding the dispositional mechanisms conducive to psychological health and wellbeing across late childhood and early adolescence
Pecularities of Hall effect in GaAs/{\delta}<Mn>/GaAs/In\timesGa1-\timesAs/GaAs (\times {\approx} 0.2) heterostructures with high Mn content
Transport properties of GaAs/{\delta}/GaAs/In\timesGa1-\timesAs/GaAs
structures containing InxGa1-xAs (\times {\approx} 0.2) quantum well (QW) and
Mn delta layer (DL) with relatively high, about one Mn monolayer (ML) content,
are studied. In these structures DL is separated from QW by GaAs spacer with
the thickness ds = 2-5 nm. All structures possess a dielectric character of
conductivity and demonstrate a maximum in the resistance temperature dependence
Rxx(T) at the temperature {\approx} 46K which is usually associated with the
Curie temperature Tc of ferromagnetic (FM) transition in DL. However, it is
found that the Hall effect concentration of holes pH in QW does not decrease
below TC as one ordinary expects in similar systems. On the contrary, the
dependence pH(T) experiences a minimum at T = 80-100 K depending on the spacer
thickness, then increases at low temperatures more strongly than ds is smaller
and reaches a giant value pH = (1-2)\cdot10^13 cm^(-2). Obtained results are
interpreted in the terms of magnetic proximity effect of DL on QW, leading to
induce spin polarization of the holes in QW. Strong structural and magnetic
disorder in DL and QW, leading to the phase segregation in them is taken into
consideration. The high pH value is explained as a result of compensation of
the positive sign normal Hall effect component by the negative sign anomalous
Hall effect component.Comment: 19 pages, 6 figure
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