69 research outputs found
Search for novel order in URu2Si2 by neutron scattering
We have made extensive reciprocal space maps in the heavy-fermion superconductor URu2Si2 using high-resolution time-of-flight single-crystal neutron diffraction to search for signs of a hidden order parameter related to the 17.5 K phase transition. Within the present sensitivity of the experiment (0.007 mu(B)/U-ion for sharp peaks), no additional features such as incommensurate structures or short-range order have been found in the (hOl) or (hhl) scattering planes. The only additional low-temperature scattering observed was the well-known tiny antiferromagnetic moment of 0.03 mu(B)/U-ion
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
Resonant magnetic exciton mode in the heavy-fermion antiferromagnet CeB6
Resonant magnetic excitations are widely recognized as hallmarks of
unconventional superconductivity in copper oxides, iron pnictides, and
heavy-fermion compounds. Numerous model calculations have related these modes
to the microscopic properties of the pair wave function, but the mechanisms
underlying their formation are still debated. Here we report the discovery of a
similar resonant mode in the non-superconducting, antiferromagnetically ordered
heavy-fermion metal CeB6. Unlike conventional magnons, the mode is
non-dispersive, and its intensity is sharply concentrated around a wave vector
separate from those characterizing the antiferromagnetic order. The magnetic
intensity distribution rather suggests that the mode is associated with a
coexisting order parameter of the unusual antiferro-quadrupolar phase of CeB6,
which has long remained "hidden" to the neutron-scattering probes. The mode
energy increases continuously below the onset temperature for
antiferromagnetism, in parallel to the opening of a nearly isotropic spin gap
throughout the Brillouin zone. These attributes bear strong similarity to those
of the resonant modes observed in unconventional superconductors below their
critical temperatures. This unexpected commonality between the two disparate
ground states indicates the dominance of itinerant spin dynamics in the ordered
low-temperature phases of CeB6 and throws new light on the interplay between
antiferromagnetism, superconductivity, and "hidden" order parameters in
correlated-electron materials
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
Colossal thermomagnetic response in the exotic superconductor URu2Si2
When a superconductor is heated above its critical temperature ,
macroscopic coherence vanishes, leaving behind droplets of thermally
fluctuating Cooper pair. This superconducting fluctuation effect above
has been investigated for many decades and its influence on the transport,
thermoelectric and thermodynamic quantities in most superconductors is well
understood by the standard Gaussian fluctuation theories. The transverse
thermoelectric (Nernst) effect is particularly sensitive to the fluctuations,
and the large Nernst signal found in the pseudogap regime of the underdoped
high- cuprates has raised much debate on its connection to the origin of
superconductivity. Here we report on the observation of a colossal Nernst
signal due to the superconducting fluctuations in the heavy-fermion
superconductor URuSi. The Nernst coefficient is enhanced by as large as
one million times over the theoretically expected value within the standard
framework of superconducting fluctuations. This, for the first time in any
known material, results in a sizeable thermomagnetic figure of merit
approaching unity. Moreover, contrary to the conventional wisdom, the
enhancement in the Nernst signal is more significant with the reduction of the
impurity scattering rate. This anomalous Nernst effect intimately reflects the
highly unusual superconducting state embedded in the so-called hidden-order
phase of URuSi. The results invoke possible chiral or Berry-phase
fluctuations originated from the topological aspect of this superconductor,
which are associated with the effective magnetic field intrinsically induced by
broken time-reversal symmetry of the superconducting order parameter.Comment: Original version. Accepted for publication in Nature Physic
LINEAR AND NONLINEAR AC SUSCEPTIBILITIES OF THE SPIN-GLASS EU0.4SR0.6S
WOS: A1992JG75000015The linear and non-linear susceptibilities of a spin-glass Eu0.4Sr0.6S system have been measured in the temperature range from 1.2 to 4.2 K, for frequencies between 1 and 234 Hz. The freezing temperature T(f)(omega --> 0) of the system is found to be 1.60 +/- 0.02 K, as determined from the Cole-Cole analysis using the linear-susceptibility data. In the vicinity of T(f) the non-linear susceptibility varies much more strongly with temperature than the linear susceptibility does. The third-harmonic results show the beginning of a power-law divergence chi-3 is-proportional-to epsilon(-gamma), where epsilon = (T- T(f))/T(f), and the curve quickly becomes rounded. This behaviour suggests a cooperative phenomenon that is greatly modified by strong dynamical and non-equilibrium effects as T(f) is approached. The critical exponent gamma is estimated to be 2.35 +/- 0.20 in the limited temperature region where a fit was obtained. In addition it is experimentally found that even a small DC field has a large effect on the third harmonic
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