arXiv subjects

Didier Jaccard

Publications and source records attributed to Didier Jaccard.

9 recordsLinked to original sources

arXiv2018

The Dominant Role of Critical Valence Fluctuations on High $T_{\rm c}$ 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 CeCu$_2$Si$_2$ away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu$_2$Si$_2$ 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 $T_{\rm cr}$. 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 $T_{\rm c}$ and the valence instability parameter $T_{\rm cr}$, 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 $T_{\rm c}$ of a given sample is mainly controlled by the compound's $T_{\rm cr}$ 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.

cond-mat.str-el↗

arXiv2017

Heavy-Fermion Superconductivity in CeAg$_2$Si$_2$ --Interplay of Spin and Valence Fluctuations--

We present the pressure-temperature phase diagram of the antiferromagnet CeAg$_2$Si$_2$ established via resistivity and calorimetry measurements under quasi-hydrostatic conditions up to 22.5~GPa. With increasing pressure, the Néel temperature [$T_{\mathrm{N}}(p=0)=8.6$~K] slowly increases up to $T_{\mathrm{N}}=13.4$~K at 9.4~GPa and then vanishes abruptly at the magnetic critical pressure $p_{\mathrm{c}}\sim13$~GPa. For the first time, heavy fermion superconductivity is observed in CeAg$_2$Si$_2$ . Superconductivity emerges at $\sim 11$~GPa and persists over roughly 10~GPa. Partial- and bulk-transition temperatures are highest at $p=16$~GPa, with a maximal $T_{\mathrm{c}^{\rm bulk}}=1.25$~K. In the pressure region of superconductivity, Kondo and crystal-field splitting energies become comparable and resistivity exhibits clear signatures of a Ce-ion valence crossover. The crossover line is located at a rapid collapse in resistivity as function of pressure and extrapolates to a valence transition critical endpoint at critical pressure and temperature of $p_{\rm cr}\sim 17$~GPa and $T_{\rm cr}\sim-13~K$, respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg$_2$Si$_2$.

cond-mat.str-el↗

arXiv2017

High-pressure study of the ground- and superconducting-state properties of CeAu$_2$Si$_2$

The pressure-temperature-phase diagram of the new heavy-fermion superconductor CeAu$_2$Si$_2$ is markedly different from those studied previously. Indeed, superconductivity emerges, not on the verge, but deep inside the magnetic phase. In this context, we have carried out ac-calorimetry, resistivity and thermoelectric power measurements on a CeAu$_2$Si$_2$-single crystal under high-pressure. The principal novelties of this experiment are the observation of a new transition line, presumably a magnetic structure rearrangement at $T_{\mathrm{M}}^{\mathrm{mod}}$ inside the antiferromagnetic phase, and the occurrence of quantum critical behavior in resistivity linked to superconductivity. Strong non-Fermi-liquid behavior is observed around the maximum of superconductivity and enhanced scattering rates are observed close to both the emergence and the maximum of superconductivity. Intriguingly, $T_{\mathrm{M}}^{\mathrm{mod}}$ almost coincides with the onset of the superconducting transition over a broad pressure range, where mysteriously Tc increases with the strengthening of magnetism. A comparison of the features in CeAu$_2$Si$_2$ and its parent compounds CeCu$_2$Si$_2$ and CeCu$_2$Ge$_2$ plotted as function of the unit-cell volume leads us to promote that critical fluctuations of a valence crossover play a crucial role in the superconducting paring mechanism. Our study reveals new intriguing features of magnetism and illustrates a complex interplay between magnetism, quantum criticality and superconductivity.

cond-mat.str-el↗

arXiv2006

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.

cond-mat.supr-con↗

arXiv2006

Controlled Stark shifts in Er$^{3+}$-doped crystalline and amorphous waveguides for quantum state storage

We present measurements of the linear Stark effect on the $^{4}$I$_{15/2} \to$ $^{4}$I$_{13/2}$ transition in an Er$^{3+}$-doped proton-exchanged LiNbO$_{3}$ crystalline waveguide and an Er$^{3+}$-doped silicate fiber. The measurements were made using spectral hole burning techniques at temperatures below 4 K. We measured an effective Stark coefficient $(Δμ_{e}χ)/(h)=25\pm1$kHz/Vcm$^{-1}$ in the crystalline waveguide and $(\bar{Δμ_{e}}χ)/(h)=15\pm1$kHz/Vcm$^{-1}$ in the silicate fiber. These results confirm the potential of Erbium doped waveguides for quantum state storage based on controlled reversible inhomogeneous broadening.

quant-ph↗

arXiv2006

Investigations of Optical Coherence Properties in an Erbium-doped Silicate Fiber for Quantum State Storage

We studied optical coherence properties of the 1.53 $μ$m telecommunication transition in an Er$^{3+}$-doped silicate optical fiber through spectral holeburning and photon echoes. We find decoherence times of up to 3.8 $μ$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$^{3+}$ 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 $^{27}$Al host nuclei. Our results show that Er$^{3+}$-doped silicate fibers are promising material candidates for quantum state storage.

quant-ph↗

arXiv2005

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.

cond-mat.supr-con↗

arXiv2005

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.

cond-mat.supr-con↗

arXiv2004

On the thermoelectricity of correlated electrons in the zero-temperature limit

The Seebeck coefficient of a metal is expected to display a linear temperature-dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1K. We put under scrutiny the magnitude of this term in different families of strongly-interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalisation contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly-correlated electron systems.

cond-mat.str-el↗