37 research outputs found
Universal Distribution of Kondo Temperatures in Dirty Metals
Kondo screening of diluted magnetic impurities in a disordered host is
studied analytically and numerically in one, two and three dimensions. It is
shown that in the T_K \to 0 limit the distribution of Kondo temperatures has a
universal form, P(T_K) \sim T_K^{-\alpha} that holds in the insulating phase
and persists in the metallic phase close to the metal insulator transition.
Moreover, the exponent \alpha depends only on the dimensionality. The most
important consequence of this result is that the T-dependence of thermodynamic
properties is smooth across the metal-insulator transition in three dimensional
systems.Comment: 4 pages, 3 figures; added referenc
Quantum criticality in inter-band superconductors
In fermionic systems with different types of quasi-particles, attractive
interactions can give rise to exotic superconducting states, as pair density
wave (PDW) superconductivity and breached pairing. In the last years the search
for these new types of ground states in cold atom and in metallic systems has
been intense. In the case of metals the different quasi-particles may be the up
and down spin bands in an external magnetic field or bands arising from
distinct atomic orbitals that coexist at a common Fermi surface. These systems
present a complex phase diagram as a function of the difference between the
Fermi wave-vectors of the different bands. This can be controlled by external
means, varying the density in the two-component cold atom system or, in a
metal, by applying an external magnetic field or pressure. Here we study the
zero temperature instability of the normal system as the Fermi wave-vectors
mismatch of the quasi-particles (bands) is reduced and find a second order
quantum phase transition to a PDW superconducting state. From the nature of the
quantum critical fluctuations close to the superconducting quantum critical
point (SQCP), we obtain its dynamic critical exponent. It turns out to be
and this allows to fully characterize the SQCP for dimensions .Comment: 5 pages, 1 figur
Evidence for time-reversal symmetry breaking of the superconducting state near twin-boundary interfaces in FeSe
Junctions and interfaces consisting of unconventional superconductors provide
an excellent experimental playground to study exotic phenomena related to the
phase of the order parameter. Not only the complex structure of unconventional
order parameters have an impact on the Josephson effects, but also may
profoundly alter the quasi-particle excitation spectrum near a junction. Here,
by using spectroscopic-imaging scanning tunneling microscopy, we visualize the
spatial evolution of the local density of states (LDOS) near twin boundaries
(TBs) of the nodal superconductor FeSe. The rotation of the
crystallographic orientation across the TB twists the structure of the
unconventional order parameter, which may, in principle, bring about a
zero-energy LDOS peak at the TB. The LDOS at the TB observed in our study, in
contrast, does not exhibit any signature of a zero-energy peak and an apparent
gap amplitude remains finite all the way across the TB. The low-energy
quasiparticle excitations associated with the gap nodes are affected by the TB
over a distance more than an order of magnitude larger than the coherence
length . The modification of the low-energy states is even more
prominent in the region between two neighboring TBs separated by a distance
. In this region the spectral weight near the Fermi level
(0.2~meV) due to the nodal quasiparticle spectrum is almost
completely removed. These behaviors suggest that the TB induces a fully-gapped
state, invoking a possible twist of the order parameter structure which breaks
time-reversal symmetry.Comment: 12 pages, 6 figure
Superconductivity induced by spark erosion in ZrZn2
We show that the superconductivity observed recently in the weak itinerant
ferromagnet ZrZn2 [C. Pfleiderer et al., Nature (London) 412, 58 (2001)] is due
to remnants of a superconducting layer induced by spark erosion. Results of
resistivity, susceptibility, specific heat and surface analysis measurements on
high-quality ZrZn2 crystals show that cutting by spark erosion leaves a
superconducting surface layer. The resistive superconducting transition is
destroyed by chemically etching a layer of 5 microns from the sample. No
signature of superconductivity is observed in rho(T) of etched samples at the
lowest current density measured, J=675 Am-2, and at T < 45 mK. EDX analysis
shows that spark-eroded surfaces are strongly Zn depleted. The simplest
explanation of our results is that the superconductivity results from an alloy
with higher Zr content than ZrZn2.Comment: Final published versio
Localized moments and the stability of antiferromagnetic order in Yb3Pt4
We present here the results of electrical resistivity {\rho}, magnetization
M, ac susceptibility \c{hi}ac', and specific heat CM measurements that have
been carried out on single crystals of Yb3Pt4 over a wide range of fields and
temperatures. The 2.4-K N\'eel temperature that is found in zero field
collapses under field to a first-order transition TN=0 at BCEP=1.85 T. In the
absence of antiferromagnetic order, the specific heat CM(T,B), the
magnetization M(T,B), and even the resistivity {\rho}(T,B) all display B/T
scaling, indicating that they are dominated by strong paramagnetic
fluctuations, where the only characteristic energy scale results from the
Zeeman splitting of an energetically isolated, Yb doublet ground state. This
paramagnetic scattering disappears with the onset of antiferromagnetic order,
revealing Fermi liquid behavior {\Delta}{\rho}=AT2 that persists up to the
antiferromagnetic phase line TN(B), but not beyond. The first-order character
of TN=0 and the ubiquity of the paramagnetic fluctuations imply that
non-Fermi-liquid behaviors are absent in Yb3Pt4. In contrast to heavy fermions
such as YbRh2Si2, Yb3Pt4 represents an extremely simple regime of f-electron
behavior where the Yb moments and conduction electrons are almost decoupled,
and where Kondo physics plays little role.Comment: 10 pages,12 figure
Magnetic field tuning of antiferromagnetic YbPt
We present measurements of the specific heat, magnetization, magnetocaloric
effect and magnetic neutron diffraction carried out on single crystals of
antiferromagnetic YbPt, where highly localized Yb moments order at
K in zero field. The antiferromagnetic order was suppressed to
by applying a field of 1.85 T in the plane.
Magnetocaloric effect measurements show that the antiferromagnetic phase
transition is always continuous for , although a pronounced step
in the magnetization is observed at the critical field in both neutron
diffraction and magnetization measurements. These steps sharpen with decreasing
temperature, but the related divergences in the magnetic susceptibility are cut
off at the lowest temperatures, where the phase line itself becomes vertical in
the field-temperature plane. As , the antiferromagnetic
transition is increasingly influenced by a quantum critical endpoint, where
ultimately vanishes in a first order phase transition.Comment: 9 pages, 6 figure
Quantum Criticality via Magnetic Branes
Holographic methods are used to investigate the low temperature limit,
including quantum critical behavior, of strongly coupled 4-dimensional gauge
theories in the presence of an external magnetic field, and finite charge
density. In addition to the metric, the dual gravity theory contains a Maxwell
field with Chern-Simons coupling. In the absence of charge, the magnetic field
induces an RG flow to an infrared AdS geometry, which is
dual to a 2-dimensional CFT representing strongly interacting fermions in the
lowest Landau level. Two asymptotic Virasoro algebras and one chiral Kac-Moody
algebra arise as {\sl emergent symmetries} in the IR. Including a nonzero
charge density reveals a quantum critical point when the magnetic field reaches
a critical value whose scale is set by the charge density. The critical theory
is probed by the study of long-distance correlation functions of the boundary
stress tensor and current. All quantities of major physical interest in this
system, such as critical exponents and scaling functions, can be computed
analytically. We also study an asymptotically AdS system whose magnetic
field induced quantum critical point is governed by a IR Lifshitz geometry,
holographically dual to a D=2+1 field theory. The behavior of these holographic
theories shares important similarities with that of real world quantum critical
systems obtained by tuning a magnetic field, and may be relevant to materials
such as Strontium Ruthenates.Comment: To appear in Lect. Notes Phys. "Strongly interacting matter in
magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A.
Schmitt, H.-U. Ye
Heat Transport and the Nature of the Order Parameter in Superconducting
Recent thermal conductivity data on the heavy fermion superconductor
have been interpreted as offering support for an model of the order
parameter as opposed to an model. In this paper, we analyze this issue
from a theoretical standpoint including the detailed effects of Fermi surface
and gap anisotropy. Our conclusion is that although current data put strong
constraints on the gap anisotropy, they cannot definitively distinguish between
these two models. Measurements on samples of varying quality could be decisive
in this regard, however.Comment: 8 pages, revtex, 15 uunencoded postscript figure
The NOX toolbox: validating the role of NADPH oxidases in physiology and disease
Reactive oxygen species (ROS) are cellular signals but also disease triggers; their relative excess (oxidative stress) or shortage (reductive stress) compared to reducing equivalents are potentially deleterious. This may explain why antioxidants fail to combat diseases that correlate with oxidative stress. Instead, targeting of disease-relevant enzymatic ROS sources that leaves physiological ROS signaling unaffected may be more beneficial. NADPH oxidases are the only known enzyme family with the sole function to produce ROS. Of the catalytic NADPH oxidase subunits (NOX), NOX4 is the most widely distributed isoform. We provide here a critical review of the currently available experimental tools to assess the role of NOX and especially NOX4, i.e. knock-out mice, siRNAs, antibodies, and pharmacological inhibitors. We then focus on the characterization of the small molecule NADPH oxidase inhibitor, VAS2870, in vitro and in vivo, its specificity, selectivity, and possible mechanism of action. Finally, we discuss the validation of NOX4 as a potential therapeutic target for indications including stroke, heart failure, and fibrosis