75 research outputs found
Valence Instability and Superconductivity in Heavy Fermion Systems
Many cerium-based heavy fermion (HF) compounds have pressure-temperature
phase diagrams in which a superconducting region extends far from a magnetic
quantum critical point. In at least two compounds, CeCu2Si2 and CeCu2Ge2, an
enhancement of the superconducting transition temperature was found to coincide
with an abrupt valence change, with strong circumstantial evidence for pairing
mediated by critical valence, or charge transfer, fluctuations. This pairing
mechanism, and the valence instability, is a consequence of a f-c Coulomb
repulsion term U_fc in the hamiltonian. While some non-superconducting Ce
compounds show a clear first order valence instability, analogous to the Ce
alpha-gamma transition, we argue that a weakly first order valence transition
may be a general feature of Ce-based HF systems, and both magnetic and critical
valence fluctuations may be responsible for the superconductivity in these
systems.Comment: 11 pages, 16 figure
Valence fluctuation mediated superconductivity in CeCu2Si2
It has been proposed that there are two types of superconductivity in
CeCu2Si2, mediated by spin fluctuations at ambient pressure, and by critical
valence fluctuations around a charge instability at a pressure P_v \simeq 4.5
GPa. We present in detail some of the unusual features of this novel type of
superconducting state, including the coexistence of superconductivity and huge
residual resistivity of the order of the Ioffe-Regel limit, large and pressure
dependent resistive transition widths in a single crystal measured under
hydrostatic conditions, asymmetric pressure dependence of the specific heat
jump shape, unrelated to the resistivity width, and negative temperature
dependence of the normal state resistivity below 10 K at very high pressure.Comment: 4 pages, 4 figures; Proceedings SCES '0
Evolution of the specific-heat anomaly of the high-temperature superconductor YBa2Cu3O7 under influence of doping through application of pressure up to 10 GPa
The evolution of the specific-heat anomaly in the overdoped range of a single
crystal of the high-temperature superconductor YBa2Cu3O7 has been studied under
influence of pressure up to 10 GPa, using AC calorimetry in a Bridgman-type
pressure cell. We show that the specific-heat jump as well as the bulk Tc are
reduced with increasing pressure in accordance with a simple charge-transfer
model. This new method enables us through pressure-induced charge transfer to
study the doping dependence of the superconducting transition, as well as the
evolution of the superconducting condensation energy on a single stoichometric
sample without adding atomic disorder.Comment: final version: J. Phys.: Condens. Matter 17 (2005) 4135-414
The Dominant Role of Critical Valence Fluctuations on High Superconductivity in Heavy Fermions
Despite almost 40 years of research, the origin of heavy-fermion
superconductivity is still strongly debated. Especially, the pressure-induced
enhancement of superconductivity in CeCuSi away from the magnetic
breakdown is not sufficiently taken into consideration. As recently reported in
CeCuSi and several related compounds, optimal superconductivity occurs
at the pressure of a valence crossover, which arises from a virtual critical
end point at negative temperature . In this context, we did a
meticulous analysis of a vast set of top-quality high-pressure electrical
resistivity data of several Ce-based heavy fermion compounds. The key novelty
is the salient correlation between the superconducting transition temperature
and the valence instability parameter , which is in
line with theory of enhanced valence fluctuations. Moreover, it is found that,
in the pressure region of superconductivity, electrical resistivity is governed
by the valence crossover, which most often manifests in scaling behavior. We
develop the new idea that the optimum superconducting of a given
sample is mainly controlled by the compound's and limited by
non-magnetic disorder. In this regard, the present study provides compelling
evidence for the crucial role of critical valence fluctuations in the formation
of Cooper pairs in Ce-based heavy fermion superconductors besides the
contribution of spin fluctuations near magnetic quantum critical points, and
corroborates a plausible superconducting mechanism in strongly correlated
electron systems in general.Comment: Supplementary Material follows after the bibliograph
Probing the phase diagram of CeRu_2Ge_2 by thermopower at high pressure
The temperature dependence of the thermoelectric power, S(T), and the
electrical resistivity of the magnetically ordered CeRu_2Ge_2 (T_N=8.55 K and
T_C=7.40 K) were measured for pressures p < 16 GPa in the temperature range 1.2
K < T < 300 K. Long-range magnetic order is suppressed at a p_c of
approximately 6.4 GPa. Pressure drives S(T) through a sequence of temperature
dependences, ranging from a behaviour characteristic for magnetically ordered
heavy fermion compounds to a typical behaviour of intermediate-valent systems.
At intermediate pressures a large positive maximum develops above 10 K in S(T).
Its origin is attributed to the Kondo effect and its position is assumed to
reflect the Kondo temperature T_K. The pressure dependence of T_K is discussed
in a revised and extended (T,p) phase diagram of CeRu_2Ge_2.Comment: 7 pages, 6 figure
Signatures of valence fluctuations in CeCu2Si2 under high pressure
Simultaneous resistivity and a.c.-specific heat measurements have been
performed under pressure on single crystalline CeCu2Si2 to over 6 GPa in a
hydrostatic helium pressure medium. A series of anomalies were observed around
the pressure coinciding with a maximum in the superconducting critical
temperature, . These anomalies can be linked with an abrupt change
of the Ce valence, and suggest a second quantum critical point at a pressure
GPa, where critical valence fluctuations provide the
superconducting pairing mechanism, as opposed to spin fluctuations at ambient
pressure. Such a valence instability, and associated superconductivity, is
predicted by an extended Anderson lattice model with Coulomb repulsion between
the conduction and f-electrons. We explain the T-linear resistivity found at
in this picture, while other anomalies found around can be
qualitatively understood using the same model.Comment: Submitted to Phys. Rev.
Controlled Stark shifts in Er-doped crystalline and amorphous waveguides for quantum state storage
We present measurements of the linear Stark effect on the I I transition in an Er-doped proton-exchanged
LiNbO crystalline waveguide and an Er-doped silicate fiber. The
measurements were made using spectral hole burning techniques at temperatures
below 4 K. We measured an effective Stark coefficient
kHz/Vcm in the crystalline waveguide and
kHz/Vcm in the silicate fiber.
These results confirm the potential of Erbium doped waveguides for quantum
state storage based on controlled reversible inhomogeneous broadening.Comment: 4 pages, 2 figures v2. typo in formula correcte
The Dominant Role of Critical Valence Fluctuations on High Tc Superconductivity in Heavy Fermions
Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu2Si2 away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu2Si2 and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature Tcr. In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature Tc and the valence instability parameter Tcr, which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting Tc of a given sample is mainly controlled by the compound’s Tcr and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general
Investigations of Optical Coherence Properties in an Erbium-doped Silicate Fiber for Quantum State Storage
We studied optical coherence properties of the 1.53 m telecommunication
transition in an Er-doped silicate optical fiber through spectral
holeburning and photon echoes. We find decoherence times of up to 3.8 s at
a magnetic field of 2.2 Tesla and a temperature of 150 mK. A strong
magnetic-field dependent optical dephasing was observed and is believed to
arise from an interaction between the electronic Er spin and the
magnetic moment of tunneling modes in the glass. Furthermore, we observed
fine-structure in the Erbium holeburning spectrum originating from
superhyperfine interaction with Al host nuclei. Our results show that
Er-doped silicate fibers are promising material candidates for quantum
state storage
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