72 research outputs found
Kondo resonance narrowing in d- and f-electron systems
By developing a simple scaling theory for the effect of Hund's interactions
on the Kondo effect, we show how an exponential narrowing of the Kondo
resonance develops in magnetic ions with large Hund's interaction. Our theory
predicts an exponential reduction of the Kondo temperature with spin S of the
Hund's coupled moment, a little-known effect first observed in d-electron
alloys in the 1960's, and more recently encountered in numerical calculations
on multi-band Hubbard models with Hund's interactions. We discuss the
consequences of Kondo resonance narrowing for the Mott transition in d-band
materials, particularly iron pnictides, and the narrow ESR linewidth recently
observed in ferromagnetically correlated f-electron materials.Comment: 4 pages, 3 figure
Magnetism and d-wave superconductivity on the half-filled square lattice with frustration
The role of frustration and interaction strength on the half-filled Hubbard
model is studied on the square lattice with nearest and next-nearest neighbour
hoppings t and t' using the Variational Cluster Approximation (VCA). At
half-filling, we find two phases with long-range antiferromagnetic (AF) order:
the usual Neel phase, stable at small frustration t'/t, and the so-called
collinear (or super-antiferromagnet) phase with ordering wave-vector
or , stable for large frustration. These are separated by a phase with
no detectable long-range magnetic order. We also find the d-wave
superconducting (SC) phase (), which is favoured by frustration if
it is not too large. Intriguingly, there is a broad region of coexistence where
both AF and SC order parameters have non-zero values. In addition, the physics
of the metal-insulator transition in the normal state is analyzed. The results
obtained with the help of the VCA method are compared with the large-U
expansion of the Hubbard model and known results for the frustrated J1-J2
Heisenberg model. These results are relevant for pressure studies of undoped
parents of the high-temperature superconductors: we predict that an insulator
to d-wave SC transition may appear under pressure.Comment: 12 pages, 10 figure
Quantum Criticality without Tuning in the Mixed Valence Compound beta-YbAlB4
Fermi liquid theory, the standard theory of metals, has been challenged by a
number of observations of anomalous metallic behavior found in the vicinity of
a quantum phase transition. The breakdown of the Fermi liquid is accomplished
by fine-tuning the material to a quantum critical point using a control
parameter such as the magnetic field, pressure, or chemical composition. Our
high precision magnetization measurements of the ultrapure f-electron based
superconductor {\beta}-YbAlB4 demonstrate a scaling of its free energy
indicative of zero-field quantum criticality without tuning in a metal. The
breakdown of Fermi-liquid behavior takes place in a mixed-valence state, in
sharp contrast with other known examples of quantum critical f-electron systems
that are magnetic Kondo lattice systems with integral valence.Comment: 26 pages, 7 figures including supporting online matelial
{\beta}-YbAlB4: a critical nodal metal
We propose a model for the intrinsic quantum criticality of {\beta}-YbAlB4,
in which a vortex in momentum space gives rise to a new type of Fermi surface
singularity. The unquenched angular momentum of the |J = 7/2,m_J = \pm5/2> Yb
4f-states generates a momentum-space line defect in the hybridization between
4f and conduction electrons, leading to a quasi-two dimensional Fermi surface
with a k\perp4 dispersion and a singular density of states proportional to
E^{-1/2}. We discuss the implications of this line-node in momentum space for
our current understanding of quantum criticality and its interplay with
topology
High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations
Structural evolution of a prospective hydrogen storage material, ammonia
borane NH3BH3, has been studied at high pressures up to 12 GPa and at low
temperatures by synchrotron powder diffraction. At 293 K and above 1.1 GPa a
disordered I4mm structure reversibly transforms into a new ordered phase. Its
Cmc21 structure was solved from the diffraction data, the positions of N and B
atoms and the orientation of NH3 and BH3 groups were finally assigned with the
help of density functional theory calculations. Group-theoretical analysis
identifies a single two-component order parameter, combining ordering and
atomic displacement mechanisms, which link an orientationally disordered parent
phase I4mm with ordered distorted Cmc21, Pmn21 and P21 structures. We propose a
generic phase diagram for NH3BH3, mapping three experimentally found and one
predicted (P21) phases as a function of temperature and pressure, along with
the evolution of the corresponding structural distortions. Ammonia borane
belongs to the class of improper ferroelastics and we show that both
temperature- and pressure-induced phase transitions can be driven to be of the
second order. The role of N-H...H-B dihydrogen bonds and other intermolecular
interactions in the stability of NH3BH3 polymorphs is examined.Comment: 23 pages, 7 figure
Tuning of magnetic quantum criticality in artificial Kondo superlattice CeRhIn5/YbRhIn5
The effects of reduced dimensions and the interfaces on antiferromagnetic
quantum criticality are studied in epitaxial Kondo superlattices, with
alternating layers of heavy-fermion antiferromagnet CeRhIn and 7 layers
of normal metal YbRhIn. As is reduced, the Kondo coherence temperature
is suppressed due to the reduction of effective Kondo screening. The N\'{e}el
temperature is gradually suppressed as decreases and the quasiparticle mass
is strongly enhanced, implying dimensional control toward quantum criticality.
Magnetotransport measurements reveal that a quantum critical point is reached
for superlattice by applying small magnetic fields. Remarkably, the
anisotropy of the quantum critical field is opposite to the expectations from
the magnetic susceptibility in bulk CeRhIn, suggesting that the Rashba
spin-orbit interaction arising from the inversion symmetry breaking at the
interface plays a key role for tuning the quantum criticality in the
two-dimensional Kondo lattice.Comment: Main text: 5 pages, 4 figures; Supplemental material:6 pages, 3
figures. Accepted for publication in Physical Review Letter
Topological metal behavior in GeBi2Te4 single crystals
The metallic character of the GeBi2Te4 single crystals is probed using a
combination of structural and physical properties measurements, together with
density functional theory (DFT) calculations. The structural study shows
distorted Ge coordination polyhedra, mainly of the Ge octahedra. This has a
major impact on the band structure, resulting in bulk metallic behavior of
GeBi2Te4, as indicated by DFT calculations. Such calculations place GeBi2Te4 in
a class of a few known non-trivial topological metals, and explains why an
observed Dirac point lies below the Fermi energy at about -0.12eV. A
topological picture of GeBi2Te4 is confirmed by the observation of surface
state modulations by scanning tunneling microscopy (STM).Comment: 10 pages, 8 figure
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