3,376 research outputs found
Enhancement of superconductivity near the ferromagnetic quantum critical point in UCoGe
We report a high-pressure single crystal study of the superconducting
ferromagnet UCoGe. Ac-susceptibility and resistivity measurements under
pressures up to 2.2 GPa show ferromagnetism is smoothly depressed and vanishes
at a critical pressure GPa. Near the ferromagnetic critical point
superconductivity is enhanced. Upper-critical field measurements under pressure
show attains remarkably large values, which provides solid evidence
for spin-triplet superconductivity over the whole pressure range. The obtained
phase diagram reveals superconductivity is closely connected to a
ferromagnetic quantum critical point hidden under the superconducting `dome'.Comment: 4 pages, 3 figures; accepted for publication in PR
Kondo hybridisation and the origin of metallic states at the (001) surface of SmB6
SmB6, a well-known Kondo insulator, has been proposed to be an ideal
topological insulator with states of topological character located in a clean,
bulk electronic gap, namely the Kondo hybridisation gap. Seeing as the Kondo
gap arises from many body electronic correlations, this would place SmB6 at the
head of a new material class: topological Kondo insulators. Here, for the first
time, we show that the k-space characteristics of the Kondo hybridisation
process is the key to unravelling the origin of the two types of metallic
states observed directly by ARPES in the electronic band structure of
SmB6(001). One group of these states is essentially of bulk origin, and cuts
the Fermi level due to the position of the chemical potential 20 meV above the
lowest lying 5d-4f hybridisation zone. The other metallic state is more
enigmatic, being weak in intensity, but represents a good candidate for a
topological surface state. However, before this claim can be substantiated by
an unequivocal measurement of its massless dispersion relation, our data raises
the bar in terms of the ARPES resolution required, as we show there to be a
strong renormalisation of the hybridisation gaps by a factor 2-3 compared to
theory, following from the knowledge of the true position of the chemical
potential and a careful comparison with the predictions from recent
LDA+Gutzwiler calculations. All in all, these key pieces of evidence act as
triangulation markers, providing a detailed description of the electronic
landscape in SmB6, pointing the way for future, ultrahigh resolution ARPES
experiments to achieve a direct measurement of the Dirac cones in the first
topological Kondo insulator.Comment: 9 pages, 4 Figures and supplementary material (including Movies and
CORPES13 "best prize" poster
Superconductivity in heavy-fermion U(Pt,Pd)3 and its interplay with magnetism
The effect of Pd doping on the superconducting phase diagram of the
unconventional superconductor UPt3 has been measured by (magneto)resistance,
specific heat, thermal expansion and magnetostriction. Experiments on single-
and polycrystalline U(Pt1-xPdx)3 for x<= 0.006 show that the superconducting
transition temperatures of the A phase, Tc+, and of the B phase, Tc-, both
decrease, while the splitting DTc increases at a rate of 0.30(2)K/at.%Pd. We
find that DTc(x) correlates with an increase of the weak magnetic moment m(x)
upon Pd doping. This provides further evidence for Ginzburg-Landau scenarios
with magnetism as the symmetry breaking field, i.e. the 2D E representation and
the 1D odd parity model. Only for small splittings DTc is proportional to
m^2(Tc+) (DTc<= 0.05 K) as predicted. The results at larger splittings call for
Ginzburg-Landau expansions beyond 4th order. The tetracritical point in the B-T
plane persists till at least x= 0.002 for B perpendicular to c, while it is
rapidly suppressed for B||c. Upon alloying the A and B phases gain stability at
the expense of the C phase.Comment: 25 pages text (PS), 8 pages with 14 figures (PS), submitted to
Phys.Rev.
Chronology Protection in Galileon Models and Massive Gravity
Galileon models are a class of effective field theories that have recently
received much attention. They arise in the decoupling limit of theories of
massive gravity, and in some cases they have been treated in their own right as
scalar field theories with a specific nonlinearly realized global symmetry
(Galilean transformation). It is well known that in the presence of a source,
these Galileon theories admit superluminal propagating solutions, implying that
as quantum field theories they must admit a different notion of causality than
standard local Lorentz invariant theories. We show that in these theories it is
easy to construct closed timelike curves (CTCs) within the {\it naive} regime
of validity of the effective field theory. However, on closer inspection we see
that the CTCs could never arise since the Galileon inevitably becomes
infinitely strongly coupled at the onset of the formation of a CTC. This
implies an infinite amount of backreaction, first on the background for the
Galileon field, signaling the break down of the effective field theory, and
subsequently on the spacetime geometry, forbidding the formation of the CTC.
Furthermore the background solution required to create CTCs becomes unstable
with an arbitrarily fast decay time. Thus Galileon theories satisfy a direct
analogue of Hawking's chronology protection conjecture.Comment: 34 pages, no figure
Electronic structure of the candidate 2D Dirac semimetal SrMnSb2: a combined experimental and theoretical study
SrMnSb is suggested to be a magnetic topological semimetal. It contains
square, 2D Sb planes with non-symmorphic crystal symmetries that could protect
band crossings, offering the possibility of a quasi-2D, robust Dirac semi-metal
in the form of a stable, bulk (3D) crystal. Here, we report a combined and
comprehensive experimental and theoretical investigation of the electronic
structure of SrMnSb, including the first ARPES data on this compound.
SrMnSb possesses a small Fermi surface originating from highly 2D, sharp
and linearly dispersing bands (the Y-states) around the (0,/a)-point in
-space. The ARPES Fermi surface agrees perfectly with that from
bulk-sensitive Shubnikov de Haas data from the same crystals, proving the
Ystates to be responsible for electrical conductivity in SrMnSb. DFT and
tight binding (TB) methods are used to model the electronic states, and both
show good agreement with the ARPES data. Despite the great promise of the
latter, both theory approaches show the Y-states to be gapped above E,
suggesting trivial topology. Subsequent analysis within both theory approaches
shows the Berry phase to be zero, indicating the non-topological character of
the transport in SrMnSb, a conclusion backed up by the analysis of the
quantum oscillation data from our crystals.Comment: 26 pages, 10 figures, revised submission to SciPost after including
changes requested by referees. All referee reports are open and can be viewed
here: https://scipost.org/submissions/1711.07165v2
Inflection point in the magnetic field dependence of the ordered moment of URu2Si2 observed by neutron scattering in fields up to 17 T
We have measured the magnetic field dependence of the ordered
antiferromagnetic moment and the magnetic excitations in the heavy-fermion
superconductor URu2Si2 for fields up to 17 Tesla applied along the tetragonal c
axis, using neutron scattering. The decrease of the magnetic intensity of the
tiny moment with increasing field does not follow a simple power law, but shows
a clear inflection point, indicating that the moment disappears first at the
metamagnetic transition at ~40 T. This suggests that the moment m is connected
to a hidden order parameter Phi which belongs to the same irreducible
representation breaking time-reversal symmetry. The magnetic excitation gap at
the antiferromagnetic zone center Q=(1,0,0) increases continuously with
increasing field, while that at Q=(1.4,0,0) is nearly constant. This field
dependence is opposite to that of the gap extracted from specific-heat data.Comment: 10 pages, 5 figures, submitted to PR
Suppression of Superconductivity in Single Crystals of UPt_3 by Pd Substitution
The suppression of superconductivity by substitution effects has been
measured in high quality single crystals of U(Pt_{1-x} Pd_x)_3 with 0 <= x <=
0.002. While the superconducting transition temperature T_c varies linearly
with residual resistivity r_0, consistent with pair-breaking by impurity
potential scattering, the rate of suppression of T_c with r_0 is much larger
for Pd substitution than for other impurity substitutions or by increased
defect density. This effect is correlated with an increase in the inelastic
scattering coefficient, and may be related to Pd-induced changes in the
magnetic fluctuation spectrum.Comment: 12 page in manuscript, plus 4 figure
Analog gravity from field theory normal modes?
We demonstrate that the emergence of a curved spacetime ``effective
Lorentzian geometry'' is a common and generic result of linearizing a field
theory around some non-trivial background. This investigation is motivated by
considering the large number of ``analog models'' of general relativity that
have recently been developed based on condensed matter physics, and asking
whether there is something more fundamental going on. Indeed, linearization of
a classical field theory (a field theoretic ``normal mode analysis'') results
in fluctuations whose propagation is governed by a Lorentzian-signature curved
spacetime ``effective metric''. For a single scalar field, this procedure
results in a unique effective metric, which is quite sufficient for simulating
kinematic aspects of general relativity (up to and including Hawking
radiation). Quantizing the linearized fluctuations, the one-loop effective
action contains a term proportional to the Einstein--Hilbert action, suggesting
that while classical physics is responsible for generating an ``effective
geometry'', quantum physics can be argued to induce an ``effective dynamics''.
The situation is strongly reminiscent of Sakharov's ``induced gravity''
scenario, and suggests that Einstein gravity is an emergent low-energy
long-distance phenomenon that is insensitive to the details of the high-energy
short-distance physics. (We mean this in the same sense that hydrodynamics is a
long-distance emergent phenomenon, many of whose predictions are insensitive to
the short-distance cutoff implicit in molecular dynamics.)Comment: Revtex 4 (beta 5); 12 pages in single-column forma
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