190 research outputs found
Design requirements for laminar airflow clean rooms and devices
Laminar airflow and airborne contamination control concepts with clean room specifications and laminar flow facility design
Critical spin-flip scattering at the helimagnetic transition of MnSi
We report spherical neutron polarimetry (SNP) and discuss the spin-flip
scattering cross sections as well as the chiral fraction close to the
helimagnetic transition in MnSi. For our study, we have developed a
miniaturised SNP device that allows fast data collection when used in small
angle scattering geometry with an area detector. Critical spin-flip scattering
is found to be governed by chiral paramagnons that soften on a sphere in
momentum space. Carefully accounting for the incoherent spin-flip background,
we find that the resulting chiral fraction decreases gradually above the
helimagnetic transition reflecting a strongly renormalised chiral correlation
length with a temperature dependence in excellent quantitative agreement with
the Brazovskii theory for a fluctuation-induced first order transition.Comment: 5 pages, 3 figure
Neon Lights Up a Controversy: the Solar Ne/O Abundance
The standard solar model was so reliable that it could predict the existence
of the massive neutrino. Helioseismology measurements were so precise that they
could determine the depth of the convection zone. This agreement between theory
and observation was the envy of all astrophysics -- until recently when
sophisticated three-dimensional hydrodynamic calculations of the solar
atmosphere reduced the metal content by a factor of almost two. Antia & Basu
(2005) suggested that a higher value of the solar neon abundance, Ne/O = 0.52,
would resolve this controversy. Drake & Testa (2005) presented strong evidence
in favor of this idea from a sample of 21 Chandra stars with enhanced values of
the neon abundance, Ne/O = 0.41. In this paper, we have analyzed solar active
region spectra from the archive of the Flat Crystal Spectrometer on Solar
Maximum Mission, a NASA mission from the 1980s, as well as full-Sun spectra
from the pioneering days of X-ray astronomy in the 1960s. These data seem
consistent with the standard neon-to-oxygen abundance value, Ne/O = 0.15
(Grevesse & Sauval 1998). If these results prove to be correct, than the
enhanced-neon hypothesis will not resolve the current controversy.Comment: submitted to ApJ Letter
Energy-resolved inelastic electron scattering off a magnetic impurity
We study inelastic scattering of energetic electrons off a Kondo impurity. If
the energy E of the incoming electron (measured from the Fermi level) exceeds
significantly the Kondo temperature T_K, then the differential inelastic
cross-section \sigma (E,w), i.e., the cross-section characterizing scattering
of an electron with a given energy transfer w, is well-defined. We show that
\sigma (E,w) factorizes into two parts. The E-dependence of \sigma (E,w) is
logarithmically weak and is due to the Kondo renormalization of the effective
coupling. We are able to relate the w-dependence to the spin-spin correlation
function of the magnetic impurity. Using this relation, we demonstrate that in
the absence of magnetic field the dynamics of the impurity spin causes the
electron scattering to be inelastic at any temperature. Quenching of the spin
dynamics by an applied magnetic field results in a finite elastic component of
the electron scattering cross-section. The differential scattering
cross-section may be extracted from the measurements of relaxation of hot
electrons injected in conductors containing localized spins.Comment: 15 pages, 9 figures; final version as published, minor changes,
reference adde
Critical conductance of a one-dimensional doped Mott insulator
We consider the two-terminal conductance of a one-dimensional Mott insulator
undergoing the commensurate-incommensurate quantum phase transition to a
conducting state. We treat the leads as Luttinger liquids. At a specific value
of compressibility of the leads, corresponding to the Luther-Emery point, the
conductance can be described in terms of the free propagation of
non-interacting fermions with charge e/\sqrt{2}. At that point, the temperature
dependence of the conductance across the quantum phase transition is described
by a Fermi function. The deviation from the Luther-Emery point in the leads
changes the temperature dependence qualitatively. In the metallic state, the
low-temperature conductance is determined by the properties of the leads, and
is described by the conventional Luttinger liquid theory. In the insulating
state, conductance occurs via activation of e/\sqrt{2} charges, and is
independent of the Luttinger liquid compressibility.Comment: 13 pages, 3 figures. Published versio
Thermodynamic Properties of the One-Dimensional Extended Quantum Compass Model in the Presence of a Transverse Field
The presence of a quantum critical point can significantly affect the
thermodynamic properties of a material at finite temperatures. This is
reflected, e.g., in the entropy landscape S(T; c) in the vicinity of a quantum
critical point, yielding particularly strong variations for varying the tuning
parameter c such as magnetic field. In this work we have studied the
thermodynamic properties of the quantum compass model in the presence of a
transverse field. The specific heat, entropy and cooling rate under an
adiabatic demagnetization process have been calculated. During an adiabatic
(de)magnetization process temperature drops in the vicinity of a field-induced
zero-temperature quantum phase transitions. However close to field-induced
quantum phase transitions we observe a large magnetocaloric effect
Thermal Conductivity of Spin-1/2 Chains
We study the low-temperature transport properties of clean one-dimensional
spin-1/2 chains coupled to phonons. Due to the presence of approximate
conservation laws, the heat current decays very slowly giving rise to an
exponentially large heat conductivity, . As a result of an
interplay of Umklapp scattering and spinon-phonon coupling, the characteristic
energy scale turns out to be of order , where is
the Debye energy, rather than the magnetic exchange interaction -- in
agreement with recent measurements in SrCuO compounds. A large magnetic field
strongly affects the heat transport by two distinct mechanisms. First, it
induces a LINEAR spinon--phonon coupling, which alters the nature of the fixed point: the elementary excitations of the system are COMPOSITE
SPINON-PHONON objects. Second, the change of the magnetization and the
corresponding change of the wave vector of the spinons strongly affects the way
in which various Umklapp processes can relax the heat current, leading to a
characteristic fractal--like spiky behavior of when plotted as a
function of magnetization at fixed T.Comment: 16 pages, RevTex4, 2 figures included; revised refs. and some useful
comments on experimental relevance. On July 12 2005, added an appendix
correcting an error in the form of the phonon propagator. The main result is
unchange
Elastocaloric determination of the phase diagram of SrRuO
One of the main developments in unconventional superconductivity in the past two decades has been the discovery that most unconventional superconductors form phase diagrams that also contain other strongly correlated states. Many systems of interest are therefore close to more than one instability, and tuning between the resultant ordered phases is the subject of intense research1. In recent years, uniaxial pressure applied using piezoelectric-based devices has been shown to be a particularly versatile new method of tuning, leading to experiments that have advanced our understanding of the fascinating unconventional superconductor SrRuO. Here we map out its phase diagram using high-precision measurements of the elastocaloric effect in what we believe to be the first such study including both the normal and the superconducting states. We observe a strong entropy quench on entering the superconducting state, in excellent agreement with a model calculation for pairing at the Van Hove point, and obtain a quantitative estimate of the entropy change associated with entry to a magnetic state that is observed in proximity to the superconductivity. The phase diagram is intriguing both for its similarity to those seen in other families of unconventional superconductors and for extra features unique, so far, to SrRuO
Oscillations of the magnetic polarization in a Kondo impurity at finite magnetic fields
The electronic properties of a Kondo impurity are investigated in a magnetic
field using linear response theory. The distribution of electrical charge and
magnetic polarization are calculated in real space. The (small) magnetic field
does not change the charge distribution. However, it unmasks the Kondo cloud.
The (equal) weight of the d-electron components with their magnetic moment up
and down is shifted and the compensating s-electron clouds don't cancel any
longer (a requirement for an experimental detection of the Kondo cloud). In
addition to the net magnetic polarization of the conduction electrons an
oscillating magnetic polarization with a period of half the Fermi wave length
is observed. However, this oscillating magnetic polarization does not show the
long range behavior of Rudermann-Kittel-Kasuya-Yosida oscillations because the
oscillations don't extend beyond the Kondo radius. They represent an internal
electronic structure of the Kondo impurity in a magnetic field. PACS: 75.20.Hr,
71.23.An, 71.27.+
Quantum Criticality in Heavy Fermion Metals
Quantum criticality describes the collective fluctuations of matter
undergoing a second-order phase transition at zero temperature. Heavy fermion
metals have in recent years emerged as prototypical systems to study quantum
critical points. There have been considerable efforts, both experimental and
theoretical, which use these magnetic systems to address problems that are
central to the broad understanding of strongly correlated quantum matter. Here,
we summarize some of the basic issues, including i) the extent to which the
quantum criticality in heavy fermion metals goes beyond the standard theory of
order-parameter fluctuations, ii) the nature of the Kondo effect in the quantum
critical regime, iii) the non-Fermi liquid phenomena that accompany quantum
criticality, and iv) the interplay between quantum criticality and
unconventional superconductivity.Comment: (v2) 39 pages, 8 figures; shortened per the editorial mandate; to
appear in Nature Physics. (v1) 43 pages, 8 figures; Non-technical review
article, intended for general readers; the discussion part contains more
specialized topic
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