71 research outputs found
Visualizing the Formation of the Kondo Lattice and the Hidden Order in URu2Si2
Heavy electronic states originating from the f atomic orbitals underlie a
rich variety of quantum phases of matter. We use atomic scale imaging and
spectroscopy with the scanning tunneling microscope (STM) to examine the novel
electronic states that emerge from the uranium f states in URu2Si2. We find
that as the temperature is lowered, partial screening of the f electrons' spins
gives rise to a spatially modulated Kondo-Fano resonance that is maximal
between the surface U atoms. At T=17.5 K, URu2Si2 is known to undergo a 2nd
order phase transition from the Kondo lattice state into a phase with a hidden
order parameter. From tunneling spectroscopy, we identify a spatially
modulated, bias-asymmetric energy gap with a mean-field temperature dependence
that develops in the hidden order state. Spectroscopic imaging further reveals
a spatial correlation between the hidden order gap and the Kondo resonance,
suggesting that the two phenomena involve the same electronic states
Field Reentrance of the Hidden Order State of URu2Si2 under Pressure
Combination of neutron scattering and thermal expansion measurements under
pressure shows that the so-called hidden order phase of URu2Si2 reenters in
magnetic field when antiferromagnetism (AF) collapses at H_AF (T). Macroscopic
pressure studies of the HO-AF boundaries were realized at different pressures
via thermal expansion measurements under magnetic field using a strain gauge.
Microscopic proof at a given pressure is the reappearance of the resonance at
Q_0=(1,0,0) under field which is correlated with the collapse of the AF Bragg
reflections at Q_0.Comment: 5 pages, 6 figures, accepted for publication in J. Phys. Soc. Jp
Thermodynamic evidence for broken fourfold rotational symmetry in the hidden-order phase of URu2Si2
Despite more than a quarter century of research, the nature of the
second-order phase transition in the heavy-fermion metal URuSi remains
enigmatic. The key question is which symmetry is being broken below this
"hidden order" transition. We review the recent progress on this issue,
particularly focusing on the thermodynamic evidence from very sensitive
micro-cantilever magnetic torque measurements that the fourfold rotational
symmetry of the underlying tetragonal crystal is broken. The angle dependence
of the torque under in-plane field rotation exhibits the twofold oscillation
term, which sets in just below the transition temperature. This observation
restricts the symmetry of the hidden order parameter to the - or
-type, depending on whether the time reversal symmetry is preserved or
not.Comment: 7 pages, 5 figures, brief review article for Physica C Special Issue
on Stripes and Electronic Liquid Crystals in Strongly Correlated Systems,
updated references and added some discussio
Fermi Surfaces of Diborides: MgB2 and ZrB2
We provide a comparison of accurate full potential band calculations of the
Fermi surfaces areas and masses of MgB2 and ZrB2 with the de Haas-van Alphen
date of Yelland et al. and Tanaka et al., respectively. The discrepancies in
areas in MgB2 can be removed by a shift of sigma-bands downward with respect to
pi-bands by 0.24 eV. Comparison of effective masses lead to orbit averaged
electron-phonon coupling constants lambda(sigma)=1.3 (both orbits),
lambda(pi)=0.5. The required band shifts, which we interpret as an exchange
attraction for sigma states beyond local density band theory, reduces the
number of holes from 0.15 to 0.11 holes per cell. This makes the occurrence of
superconductivity in MgB2 a somewhat closer call than previously recognized,
and increases the likelihood that additional holes can lead to an increased Tc.Comment: 7 pages including 4 figure
Why the hidden order in URu2Si2 is still hidden - one simple answer
For more than two decades, the nonmagnetic anomaly observed around 17.5 K in
URu2Si2, has been investigated intensively. However, any kind of fingerprint
for the lattice anomaly has not been observed. Therefore, the order has been
called "the hidden order". One simple answer to why the hidden order is still
hidden is presented from the space group analysis. The second order phase
transition from I4/mmm (No. 139) to P4_2/mnm (No. 136) does not need any kind
of lattice distortion in this system, and allows the NQR frequency at Ru-site
unchanged. It is compatible with O_{xy}-type anti-ferro quadrupole ordering
with Q=(0, 0, 1). The characteristics of the hidden order are discussed based
on the local 5f^2 electron picture.Comment: Accepted for publication in J. Phys. Soc. Jpn., 4 pages, 2 figure
Precise study of the resonance at Q0=(1,0,0) in URu2Si2
New inelastic neutron scattering experiments have been performed on URu2Si2
with special focus on the response at Q0=(1,0,0), which is a clear signature of
the hidden order (HO) phase of the compound. With polarized inelastic neutron
experiments, it is clearly shown that below the HO temperature (T0 = 17.8 K) a
collective excitation (the magnetic resonance at E0 \approx 1.7 meV) as well as
a magnetic continuum co-exist. Careful measurements of the temperature
dependence of the resonance lead to the observation that its position shifts
abruptly in temperature with an activation law governed by the partial gap
opening and that its integrated intensity has a BCS-type temperature
dependence. Discussion with respect to recent theoretical development is made
Spin-orbit density wave induced hidden topological order in URu2Si2
The conventional order parameters in quantum matters are often characterized
by 'spontaneous' broken symmetries. However, sometimes the broken symmetries
may blend with the invariant symmetries to lead to mysterious emergent phases.
The heavy fermion metal URu2Si2 is one such example, where the order parameter
responsible for a second-order phase transition at Th = 17.5 K has remained a
long-standing mystery. Here we propose via ab-initio calculation and effective
model that a novel spin-orbit density wave in the f-states is responsible for
the hidden-order phase in URu2Si2. The staggered spin-orbit order 'spontaneous'
breaks rotational, and translational symmetries while time-reversal symmetry
remains intact. Thus it is immune to pressure, but can be destroyed by magnetic
field even at T = 0 K, that means at a quantum critical point. We compute
topological index of the order parameter to show that the hidden order is
topologically invariant. Finally, some verifiable predictions are presented.Comment: (v2) Substantially modified from v1, more calculation and comparison
with experiments are include
Full Relativistic Electronic Structure and Fermi Surface Sheets of the First Honeycomb-Lattice Pnictide Superconductor SrPtAs
We report full-potential density functional theory (DFT)-based {\it ab
initio} band structure calculations to investigate electronic structure
properties of the first pnictide superconductor with a honeycomb-lattice
structure: SrPtAs. As a result, electronic bands, density of states, Fermi
velocities and the topology of the Fermi surface for SrPtAs are obtained. These
quantities are discussed in comparison to the first available experimental
data. Predictions for future measurements are provided
High-Field Fermi Surface Properties in the Low Carrier Heavy Fermion Compound URu2Si2
We performed the Shubnikov-de Haas (SdH) experiments of the low carrier heavy
fermion compound URu2Si2 at high fields up to 34T and at low temperatures down
to 30mK. All main SdH branches named alpha, beta and gamma were observed for
all the measured field-directions (H // [001] -> [100], [100] -> [110] and
[001] -> [110]), indicating that these are attributed to the closed Fermi
surfaces with nearly spherical shapes. Anomalous split of branch alpha was
detected for the field along the basal plane, and the split immediately
disappears by tilting the field to [001] direction, implying a fingerprint of
the hidden order state. High field experiments reveal the complicated
field-dependence of the SdH frequencies and the cyclotron masses due to the
Zeeman spin-splitting associated with the Fermi surface reconstruction in the
hidden order state with small carrier numbers. A new SdH branch named omega
with large cyclotron mass of 25m0 was detected at high fields above 23T close
to the hidden order instabilities.Comment: 6 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp
Emergent Rank-5 'Nematic' Order in URu2Si2
Novel electronic states resulting from entangled spin and orbital degrees of
freedom are hallmarks of strongly correlated f-electron systems. A spectacular
example is the so-called 'hidden-order' phase transition in the heavy-electron
metal URu2Si2, which is characterized by the huge amount of entropy lost at
T_{HO}=17.5K. However, no evidence of magnetic/structural phase transition has
been found below T_{HO} so far. The origin of the hidden-order phase transition
has been a long-standing mystery in condensed matter physics. Here, based on a
first-principles theoretical approach, we examine the complete set of multipole
correlations allowed in this material. The results uncover that the
hidden-order parameter is a rank-5 multipole (dotriacontapole) order with
'nematic' E^- symmetry, which exhibits staggered pseudospin moments along the
[110] direction. This naturally provides comprehensive explanations of all key
features in the hidden-order phase including anisotropic magnetic excitations,
nearly degenerate antiferromagnetic-ordered state, and spontaneous
rotational-symmetry breaking.Comment: See the published version with more detailed discussion
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