455 research outputs found
Multidimensional entropy landscape of quantum criticality
The Third Law of Thermodynamics states that the entropy of any system in
equilibrium has to vanish at absolute zero temperature. At nonzero
temperatures, on the other hand, matter is expected to accumulate entropy near
a quantum critical point (QCP), where it undergoes a continuous transition from
one ground state to another. Here, we determine, based on general thermodynamic
principles, the spatial-dimensional profile of the entropy S near a QCP and its
steepest descent in the corresponding multidimensional stress space. We
demonstrate this approach for the canonical quantum critical compound
CeCu6-xAux near its onset of antiferromagnetic order. We are able to link the
directional stress dependence of S to the previously determined geometry of
quantum critical fluctuations. Our demonstration of the multidimensional
entropy landscape provides the foundation to understand how quantum criticality
nucleates novel phases such as high-temperature superconductivity.Comment: 14 pages, 4 figure
Magnetic-field enhanced aniferromagnetism in non-centrosymmetric heavy-fermion superconductor CePtSi
The effect of magnetic field on the static and dynamic spin correlations in
the non-centrosymmetric heavy-fermion superconductor CePtSi was
investigated by neutron scattering. The application of a magnetic field B
increases the antiferromagnetic (AFM) peak intensity. This increase depends
strongly on the field direction: for B[0 0 1] the intensity
increases by a factor of 4.6 at a field of 6.6 T, which corresponds to more
than a doubling of the AFM moment, while the moment increases by only 10 % for
B[1 0 0] at 5 T. This is in strong contrast to the inelastic
response near the antiferromagnetic ordering vector, where no marked field
variations are observed for B[0 0 1] up to 3.8 T. The results
reveal that the AFM state in CePtSi, which coexists with superconductivity,
is distinctly different from other unconventional superconductors.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.
Magnetic structure of Cd-doped CeCoIn5
The heavy fermion superconductor CeCoIn5 is believed to be close to a
magnetic instability, but no static magnetic order has been found. Cadmium
doping on the In-site shifts the balance between superconductivity and
antiferromagnetism to the latter with an extended concentration range where
both types of order coexist at low temperatures. We investigated the magnetic
structure of nominally 10% Cd-doped CeCoIn5, being antiferromagnetically
ordered below T_N=3 K and superconducting below T_c=1.3 K, by elastic neutron
scattering. Magnetic intensity was observed only at the ordering wave vector
Q_AF = (1/2,1/2,1/2) commensurate with the crystal lattice. Upon entering the
superconducting state the magnetic intensity seems to change only little. The
commensurate magnetic ordering in CeCo(In1-xCdx)5 is in contrast to the
incommensurate antiferromagnetic ordering observed in the closely related
compound CeRhIn5. Our results give new insights in the interplay between
superconductivity and magnetism in the family of CeTIn5 (T=Co, Rh, and Ir)
based compounds.Comment: 4 pages, 4 figure
Time-resolved collapse and revival of the Kondo state near a quantum phase transition
One of the most successful paradigms of many-body physics is the concept of
quasiparticles: excitations in strongly interacting matter behaving like weakly
interacting particles in free space. Quasiparticles in metals are very robust
objects. Yet, when a system's ground state undergoes a qualitative change at a
quantum critical point (QCP), the quasiparticles may disintegrate and give way
to an exotic quantum-fluid state of matter. The nature of this breakdown is
intensely debated, because the emergent quantum fluid dominates the material
properties up to high temperature and might even be related to the occurence of
superconductivity in some compounds. Here we trace the dynamics of
heavy-fermion quasiparticles in CeCuAu and monitor their
evolution towards the QCP in time-resolved experiments, supported by many-body
calculations. A terahertz pulse disrupts the many-body heavy-fermion state.
Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion
state recovers, with a coherence time 100 times longer than typically
associated with correlated metals. The quasiparticle weight collapses towards
the QCP, yet its formation temperature remains constant -- phenomena believed
to be mutually exclusive. Coexistence in the same experiment calls for
revisions in our view on quantum criticality.Comment: Published version, including data on CeCu6, CeCu5.9Au0.1, and
CeCu5Au1 and extended Supplementary Information. 7 pages, 4 figures,
Supplementary Information: 5 pages, 3 figure
Observation of an optical non-Fermi-liquid behavior in the heavy fermion state of YbRhSi
We report far-infrared optical properties of YbRhSi for photon
energies down to 2 meV and temperatures 0.4 -- 300 K. In the coherent heavy
quasiparticle state, a linear dependence of the low-energy scattering rate on
both temperature and photon energy was found. We relate this distinct dynamical
behavior different from that of Fermi liquid materials to the non-Fermi liquid
nature of YbRhSi which is due to its close vicinity to an
antiferromagnetic quantum critical point.Comment: 5 pages, 4 figures. submitte
Robustness of magnons near the quantum critical point in the heavy fermion superconductor CeCu2Si2
Paramagnons are supposed to provide the pairing glue for unconventional superconductors. For the heavy fermion superconductor CeCu2Si2, there is indeed good evidence from inelastic neutron scattering INS that spin fluctuations drive the superconductivity. Here, we present the INS measurement of the inelastic response of the antiferromagnetic parent compound, A type CeCu2Si2, to probe the relation to the excitations of the superconducting S type sample. We find that the dispersion is very similar in the antiferromagnetic state and in the normal state of the superconducting sample. Pronounced differences to the response in the superconducting state exist at low energies around the zone centre. These findings are in line with observations of other unconventional superconductor
Magnetic and structural quantum phase transitions in CeCu6-xAux are independent
The heavy-fermion compound CeCuAu has become a model system for
unconventional magnetic quantum criticality. For small Au concentrations , the compound undergoes a structural transition from
orthorhombic to monoclinic crystal symmetry at a temperature with
for . Antiferromagnetic order sets in
close to . To shed light on the interplay between quantum
critical magnetic and structural fluctuations we performed neutron-scattering
and thermodynamic measurements on samples with . The
resulting phase diagram shows that the antiferromagnetic and monoclinic phase
coexist in a tiny Au concentration range between and . The
application of hydrostatic and chemical pressure allows to clearly separate the
transitions from each other and to explore a possible effect of the structural
transition on the magnetic quantum critical behavior. Our measurements
demonstrate that at low temperatures the unconventional quantum criticality
exclusively arises from magnetic fluctuations and is not affected by the
monoclinic distortion.Comment: 5 pages, 3 figure
Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCuSi versus YbRhSi
In this paper the low-temperature properties of two isostructural canonical
heavy-fermion compounds are contrasted with regards to the interplay between
antiferromagnetic (AF) quantum criticality and superconductivity. For
CeCuSi, fully-gapped d-wave superconductivity forms in the vicinity of
an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum
critical point (QCP). Inelastic neutron scattering results highlight that both
quantum critical SDW fluctuations as well as Mott-type fluctuations of local
magnetic moments contribute to the formation of Cooper pairs in CeCuSi.
In YbRhSi, superconductivity appears to be suppressed at
mK by AF order ( = 70 mK). Ultra-low temperature measurements reveal a
hybrid order between nuclear and 4f-electronic spins, which is dominated by the
Yb-derived nuclear spins, to develop at slightly above 2 mK. The hybrid
order turns out to strongly compete with the primary 4f-electronic order and to
push the material towards its QCP. Apparently, this paves the way for
heavy-fermion superconductivity to form at = 2 mK. Like the pressure -
induced QCP in CeRhIn, the magnetic field - induced one in YbRhSi
is of the local Kondo-destroying variety which corresponds to a Mott-type
transition at zero temperature. Therefore, these materials form the link
between the large family of about fifty low- unconventional heavy - fermion
superconductors and other families of unconventional superconductors with
higher s, notably the doped Mott insulators of the cuprates, organic
charge-transfer salts and some of the Fe-based superconductors. Our study
suggests that heavy-fermion superconductivity near an AF QCP is a robust
phenomenon.Comment: 30 pages, 7 Figures, Accepted for publication in Philosophical
Magazin
Magnetism and superconductivity driven by identical 4 states in a heavy-fermion metal
The apparently inimical relationship between magnetism and superconductivity
has come under increasing scrutiny in a wide range of material classes, where
the free energy landscape conspires to bring them in close proximity to each
other. This is particularly the case when these phases microscopically
interpenetrate, though the manner in which this can be accomplished remains to
be fully comprehended. Here, we present combined measurements of elastic
neutron scattering, magnetotransport, and heat capacity on a prototypical heavy
fermion system, in which antiferromagnetism and superconductivity are observed.
Monitoring the response of these states to the presence of the other, as well
as to external thermal and magnetic perturbations, points to the possibility
that they emerge from different parts of the Fermi surface. This enables a
single 4 state to be both localized and itinerant, thus accounting for the
coexistence of magnetism and superconductivity.Comment: 4 pages, 4 figure
The break up of heavy electrons at a quantum critical point
The point at absolute zero where matter becomes unstable to new forms of
order is called a quantum critical point (QCP). The quantum fluctuations
between order and disorder that develop at this point induce profound
transformations in the finite temperature electronic properties of the
material. Magnetic fields are ideal for tuning a material as close as possible
to a QCP, where the most intense effects of criticality can be studied. A
previous study on theheavy-electron material found that near a
field-induced quantum critical point electrons move ever more slowly and
scatter off one-another with ever increasing probability, as indicated by a
divergence to infinity of the electron effective mass and cross-section. These
studies could not shed light on whether these properties were an artifact of
the applied field, or a more general feature of field-free QCPs. Here we report
that when Germanium-doped is tuned away from a chemically induced
quantum critical point by magnetic fields there is a universal behavior in the
temperature dependence of the specific heat and resistivity: the characteristic
kinetic energy of electrons is directly proportional to the strength of the
applied field. We infer that all ballistic motion of electrons vanishes at a
QCP, forming a new class of conductor in which individual electrons decay into
collective current carrying motions of the electron fluid.Comment: Pdf files of article available at
http://www.physics.rutgers.edu/~coleman/online/breakup.pdf, pdf file of news
and views article available at
http://www.physics.rutgers.edu/~coleman/online/nvbreakup.pd
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