arXiv subjects

G. W. Scheerer

Publications and source records attributed to G. W. Scheerer.

9 recordsLinked to original sources

arXiv2021

Isotope tuning of the superconducting dome of strontium titanate

Doped strontium titanate SrTiO$_3$ (STO) is one of the most dilute superconductors known today. The fact that superconductivity occurs at very low carrier concentrations is one of the two reasons that the pairing mechanism is not yet understood, the other is the role played by the proximity to a ferroelectric instability. In undoped STO, ferroelectric order can in fact be stabilized by substituting $^{16}$O with its heavier isotope $^{18}$O. Here we explore the superconducting properties of doped and isotope-substituted SrTi$(^{18}$O$_{y}^{16}$O$_{1-y})_{3-δ}$ for $0\le y \le 0.81$ and carrier concentrations between $6\times 10^{17}$ and $2\times 10^{20}$ cm$^{-3}$ ($δ<0.02$). We show that the superconducting $T_c$ increases when the $^{18}$O concentration is increased. For carrier concentrations around $5\times 10^{19}$~cm$^{-3}$ this $T_c$ increase amounts to almost a factor $3$, with $T_c$ as high as 580~mK for $y=0.74$. When approaching SrTi$^{18}$O$_3$ the maximum $T_c$ occurs at a much smaller carrier densities than for pure SrTi$^{16}$O$_3$. Our observations agree qualitatively with a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft mode.

cond-mat.supr-con↗

arXiv2017

Coincidence of magnetic and valence quantum critical points in CeRhIn5 under pressure

We present accurate electrical resistivity measurements along the two principle crystallographic axes of the pressure-induced heavy-fermion superconductor CeRhIn5 up to 5.63 GPa. For both directions, a valence crossover line is identified in the p-T plane and the extrapolation of this line to zero temperature coincides with the collapse of the magnetic ordering temperature. Furthermore, it is found that the p-T phase diagram of CeRhIn5 in the valence crossover region is very similar to that of CeCu2Si2. These results point to the essential role of Ce-4f electron delocalization in both destroying magnetic order and realizing superconductivity in CeRhIn5 under pressure.

cond-mat.supr-con↗

arXiv2017

URu$_2$Si$_2$ under intense magnetic fields: from hidden order to spin-density wave

A review of recent state-of-the-art pulsed field experiments performed on URu$_2$Si$_2$ under a magnetic field applied along its easy magnetic axis $\mathbf{c}$ is given. Resistivity, magnetization, magnetic susceptibility, Shubnikov-de Haas, and neutron diffraction experiments are presented, permitting to emphasize the relationship between Fermi surface reconstructions, the destruction of the hidden-order and the appearance of a spin-density wave state in a high magnetic field.

cond-mat.str-el↗

arXiv2016

Scaling behavior of temperature-dependent thermopower in CeAu2Si2 under pressure

We report a combined study of in-plane resistivity and thermopower of the pressure-induced heavy fermion superconductor CeAu2Si2 up to 27.8 GPa. It is found that thermopower follows a scaling behavior in T/T* almost up to the magnetic critical pressure pc ~ 22 GPa. By comparing with resistivity results, we show that the magnitude and characteristic temperature dependence of thermopower in this pressure range are governed by the Kondo coupling and crystal-field splitting, respectively. Below pc, the superconducting transition is preceded by a large negative thermopower minimum, suggesting a close relationship between the two phenomena. Furthermore, thermopower of a variety of Ce-based Kondo-lattices with different crystal structures follows the same scaling relation up to T/T* ~ 2.

cond-mat.str-el↗

arXiv2015

Effect of disorder on the pressure-induced superconducting state of CeAu2Si2

CeAu2Si2 is a newly discovered pressure-induced heavy fermion superconductor which shows very unusual interplay between superconductivity and magnetism under pressure. Here we compare the results of high-pressure measurements on single crystalline CeAu2Si2 samples with different levels of disorder. It is found that while the magnetic properties are essentially sample independent, superconductivity is rapidly suppressed when the residual resistivity of the sample increases. We show that the depression of bulk Tc can be well understood in terms of pair breaking by nonmagnetic disorder, which strongly suggests an unconventional pairing state in pressurized CeAu2Si2. Furthermore, increasing the level of disorder leads to the emergence of another phase transition at T* within the magnetic phase, which might be in competition with superconductivity.

cond-mat.supr-con↗

arXiv2013

Fermi surface in the hidden-order state of URu$_2$Si$_2$ under intense pulsed magnetic fields up to 81~T

We present measurements of the resistivity $ρ_{x,x}$ of URu2Si2 high-quality single crystals in pulsed high magnetic fields up to 81~T at a temperature of 1.4~K and up to 60~T at temperatures down to 100~mK. For a field \textbf{H} applied along the magnetic easy-axis \textbf{c}, a strong sample-dependence of the low-temperature resistivity in the hidden-order phase is attributed to a high carrier mobility. The interplay between the magnetic and orbital properties is emphasized by the angle-dependence of the phase diagram, where magnetic transition fields and crossover fields related to the Fermi surface properties follow a 1/$\cosθ$-law, $θ$ being the angle between \textbf{H} and \textbf{c}. For $\mathbf{H}\parallel\mathbf{c}$, a crossover defined at a kink of $ρ_{x,x}$, as initially reported in [Shishido et al., Phys. Rev. Lett. \textbf{102}, 156403 (2009)], is found to be strongly sample-dependent: its characteristic field $μ_0H^*$ varies from $\simeq20$~T in our best sample with a residual resistivity ratio RRR of $225$ to $\simeq25$~T in a sample with a RRR of $90$. A second crossover is defined at the maximum of $ρ_{x,x}$ at the sample-independent characteristic field $μ_0H_{ρ,max}^{LT}\simeq30$~T. Fourier analyzes of SdH oscillations show that $H_{ρ,max}^{LT}$ coincides with a sudden modification of the Fermi surface, while $H^*$ lies in a regime where the Fermi surface is smoothly modified. For $\mathbf{H}\parallel\mathbf{a}$, i) no phase transition is observed at low temperature and the system remains in the hidden-order phase up to 81~T, ii) quantum oscillations surviving up to 7~K are related to a new and almost-spherical orbit - for the first time observed here - at the frequency $F_λ\simeq1400$~T and associated with a low effective mass $m^*_λ=(1\pm0.5)\cdot m_0$, and iii) no Fermi surface modification occurs up to 81~T.

cond-mat.str-el↗

arXiv2012

High-field moment polarization in the ferromagnetic superconductor UCoGe

We report magnetization and magnetoresistivity measurements on the isostructural ferromagnetic superconductors UCoGe and URhGe in magnetic fields up to 60 T and temperatures from 1.5 to 80 K. At low-temperature, a moment polarization in UCoGe in a field $μ_0\mathbf{H}\parallel\mathbf{b}$ of around 50 T leads to well-defined anomalies in both magnetization and magnetoresistivity. These anomalies vanish in temperatures higher than 30-40 K, where maxima in the magnetic susceptibility and the field-induced variation of the magnetoresistivity are found. A comparison is made between UCoGe and URhGe, where a moment reorientation in a magnetic field $μ_0\mathbf{H}\parallel\mathbf{b}$ of 12 T leads to field-induced reentrant superconductivity.

cond-mat.str-el↗

arXiv2012

High frequency magnetic oscillations of the organic metal $θ$-(ET)$_4$ZnBr$_4$(C$_6$H$_4$Cl$_2$) in pulsed magnetic field of up to 81 T

De Haas-van Alphen oscillations of the organic metal $θ$-(ET)$_4$ZnBr$_4$(C$_6$H$_4$Cl$_2$) are studied in pulsed magnetic fields up to 81 T. The long decay time of the pulse allows determining reliable field-dependent amplitudes of Fourier components with frequencies up to several kiloteslas. The Fourier spectrum is in agreement with the model of a linear chain of coupled orbits. In this model, all the observed frequencies are linear combinations of the frequency linked to the basic orbit $α$ and to the magnetic-breakdown orbit $β$.

cond-mat.str-el↗

arXiv2012

Interplay of magnetism, Fermi surface reconstructions, and hidden-order in the heavy-fermion material URu$_2$Si$_2$

URu$_2$Si$_2$ is surely one of the most mysterious of the heavy-fermion compounds. Despite more than twenty years of experimental and theoretical works, the order parameter of the transition at $T_0 = 17.5$ K is still unknown. The state below $T_0$ remains called "hidden-order phase" and the stakes are still to identify the energy scales driving the system to this phase. We present new magnetoresistivity and magnetization measurements performed on very-high-quality single crystals in pulsed magnetic fields up to 60 T. We show that the transition to the hidden-order state in URu$_2$Si$_2$ is initially driven by a high-temperature crossover at around 40-50 K, which is a fingerprint of inter-site electronic correlations. In a magnetic field $\mathbf{H}$ applied along the easy-axis $\bf{c}$, the vanishing of this high-temperature scale precedes the polarization of the magnetic moments, as well as it drives the destabilization of the hidden-order phase. Strongly impurity-dependent magnetoresistivity confirms that the Fermi surface is reconstructed below $T_0$ and is strongly modified in a high magnetic field applied along $\mathbf{c}$, i.e. at a sufficiently-high magnetic polarization. The possibility of a sharp crossover in the hidden-order state controlled by a field-induced change of the Fermi surface is pointed out.

cond-mat.str-el↗