arXiv subjects

D. Braithwaite

Publications and source records attributed to D. Braithwaite.

31 records · Page 2Linked to original sources

arXiv2011

Ferromagnetism in YbCu2Si2 at high pressure

We demonstrate from detailed ac-susceptibility and calorimetry studies under hydrostatic pressure that YbCu2Si2 probably orders ferromagnetically at high pressure. The (p,H,T) phase diagram, shows that the transition temperature increases with pressure, but also with an applied magnetic field. We suggest that many ytterbium systems may show a trend towards ferromagnetism and we discuss the possible reasons for this. We also examine the implications, including the potential of YbCu2Si2 and other Yb compounds for further studies of the rich physical properties that may occur near a ferromagnetic critical point.

cond-mat.str-el↗

arXiv2010

Thermodynamic phase diagram of Fe(Se$_{0.5}$Te$_{0.5}$) single crystals up to 28 Tesla

We report on specific heat ($C_p$), transport, Hall probe and penetration depth measurements performed on Fe(Se$_{0.5}$Te$_{0.5}$) single crystals ($T_c \sim 14$ K). The thermodynamic upper critical field $H_{c2}$ lines has been deduced from $C_p$ measurements up to 28 T for both $H\|c$ and $H\|ab$, and compared to the lines deduced from transport measurements (up to 55 T in pulsed magnetic fields). We show that this {\it thermodynamic} $H_{c2}$ line presents a very strong downward curvature for $T \rightarrow T_c$ which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirm that $H_{c2}$ is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to $T/T_c \sim 0.99$ (for $H\|ab$) which is the consequence of a very small value of the coherence length $ξ_c(0) \sim 4 Å$ (and $ξ_{ab}(0) \sim 15 Å$), confirming the strong renormalisation of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [Phys. Rev. Lett. 104, 097002 (2010)]. $H_{c1}$ measurements lead to $λ_{ab}(0) = 430 \pm 50$ nm and $λ_c(0) = 1600 \pm 200$ nm and the corresponding anisotropy is approximatively temperature independent ($\sim 4$), being close to the anisotropy of $H_{c2}$ for $T\rightarrow T_c$. The temperature dependence of both $λ$ ($\propto T^2$) and the electronic contribution to the specific heat confirm the non conventional coupling mechanism in this system.

cond-mat.supr-con↗

arXiv2009

Electric Pulse Induced Resistive Switching, Electronic Phase Separation, and Possible Superconductivity in a Mott insulator

Metal-insulator transitions (MIT) belong to a class of fascinating physical phenomena, which includes superconductivity, and colossal magnetoresistance (CMR), that are associated with drastic modifications of electrical resistance. In transition metal compounds, MIT are often related to the presence of strong electronic correlations that drive the system into a Mott insulator state. In these systems the MIT is usually tuned by electron doping or by applying an external pressure. However, it was noted recently that a Mott insulator should also be sensitive to other external perturbations such as an electric field. We report here the first experimental evidence of a non-volatile electric-pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa4Se8. Our Scanning Tunneling Microscopy experiments show that this unconventional response of the system to short electric pulses arises from a nanometer scale Electronic Phase Separation (EPS) generated in the bulk material.

cond-mat.str-el↗

arXiv2008

Low-energy magnetic response of the noncentrosymmetric heavy-fermion superconductor CePt3Si studied via inelastic neutron scattering

The low-energy magnetic excitations of the noncentrosymmetric heavy-fermion superconductor CePt3Si have been measured with inelastic neutron scattering on a single crystal. Kondo-type spin fluctuations with an anisotropic wave vector dependence are observed in the paramagnetic state. These fluctuations do not survive in the antiferromagnetically ordered state below TN=2.2 K but are replaced by damped spin waves, whose dispersion is much stronger along the c-axis than in other directions. No change is observed in the excitation spectrum or the magnetic order as the system enters the superconducting state below Tc=0.7 K.

cond-mat.str-el↗

arXiv2007

Double superconducting transition in the filled skutterudite PrOs4Sb12 and sample characterizations

A thorough characterization of many samples of the filled skutterudite compound PrOs4Sb12 is provided. We find that the double superconducting transition in the specific heat Tc1~1.89K and Tc2~1.72K tends to appear in samples with a large residual resistivity ratio, large specific heat jump at the superconducting transition and with the highest absolute value of the specific heat above Tc1. However, we present evidence which casts doubt on the intrinsic nature of the double superconducting transition. The ratio of the two specific heat jumps ΔC(Tc1)/ΔC(Tc2) shows a wide range of values on crystals from different batches but also within the same batch. This ratio was strongly reduced by polishing a sample down to 120um. Remarkably, three samples exhibit a single sharp transition of ~15mK in width at Tc~1.7K. The normalized specific heat jump (C-Cnormal)/Cnormal at Tc of two of them is higher than ~32% so larger than the sum of the two specific heat jumps when a double transition exists. As an evidence of better quality, the slope in the transition is at least two time steeper. We discuss the origins of the double transition; in particular we consider, based on X-ray diffraction results, a scenario involving Pr-vacancies. The superconducting phase diagram under magnetic field of a sample with a single transition is fitted with a two-band model taking into account the good values for the gap as deduced from thermal conductivity measurements.

cond-mat.str-el↗

arXiv2006

Unconventional resistivity at the border of metallic antiferromagnetism in NiS2

We report low-temperature and high-pressure measurements of the electrical resistivity ρ(T) of the antiferromagnetic compound NiS_2 in its high-pressure metallic state. The form of ρ(T) suggests that metallic antiferromagnetism in NiS_2 is quenched at a critical pressure p_c=76+-5 kbar. Near p_c the temperature variation of ρ(T) is similar to that observed in NiS_{2-x}Se_x near the critical composition x=1 where the Neel temperature vanishes at ambient pressure. In both cases ρ(T) varies approximately as T^{1.5} over a wide range below 100 K. However, on closer analysis the resistivity exponent in NiS_2 exhibits an undulating variation with temperature not seen in NiSSe (x=1). This difference in behaviour may be due to the effects of spin-fluctuation scattering of charge carriers on cold and hot spots of the Fermi surface in the presence of quenched disorder, which is higher in NiSSe than in stoichiometric NiS_2.

cond-mat.str-el↗

arXiv2005

Coexistence of antiferromagnetism and superconductivity in CeRhIn5 under high pressure and magnetic field

We report on detailed ac calorimetry measurements under high pressure and magnetic field of CeRhIn5. Under hydrostatic pressure the antiferromagnetic order vanishes near p_c*=2 GPa due to a first order transition. Superconductivity is found for pressures above 1.5 GPa inside the magnetic ordered phase. However, the superconductivity differ from the pure homogeneous superconducting ground state above 2 GPa. The application of an external magnetic field H || ab induces a transition inside the superconducting state above pc* which is strongly related to the re-entrance of the antiferromagnetism with field. This field-induced supplementary state vanishes above the quantum critical point in this system. The analogy to CeCoIn5 is discussed.

cond-mat.str-el↗

arXiv2005

Magnetism and superconductivity of heavy fermion matter

The interplay of magnetism and unconventional superconductivity (d singlet wave or p triplet wave) in strongly correlated electronic system (SCES) is discussed with recent examples found in heavy fermion compounds. A short presentation is given on the formation of the heavy quasiparticle with the two sources of a local and intersite enhancement for the effective mass. Two cases of the coexistence or repulsion of antiferromagnetism and superconductivity are given with CeIn3 and CeCoIn5. A spectacular example is the emergence of superconductivity in relatively strong itinerant ferromagnets UGe2 and URhGe. The impact of heavy fermion matter among other SCES as organic conductor or high Tc oxide is briefly pointed out.

cond-mat.str-el↗

arXiv2004

Spin-fluctuation dominated electrical transport of Ni3Al at high pressure

We present the first study of a magnetic quantum phase transition in the itinerant-electron ferromagnet Ni3Al at high pressures. Electrical resistivity measurements in a diamond anvil cell at hydrostatic pressures up to 100 kbar and temperatures as low as 50 mK indicate that the Curie temperature collapses towards absolute zero at a critical pressure pc=82(2) kbar. Over wide ranges in pressure and temperature, both in the ferromagnetic and paramagnetic states, the temperature variation of the resistivity is found to deviate from the conventional Fermi-liquid form. We consider the extent to which this deviation can be understood in terms of a mean-field model of enhanced spin fluctuations on the border of ferromagnetism in three dimensions.

cond-mat.str-el↗

arXiv2004

Superconducting phase diagram of the filled skuterrudite PrOs4Sb12

We present new measurements of the specific heat of the heavy fermion superconductor PrOs4Sb12, on a sample which exhibits two sharp distinct anomalies at Tc1= 1.89K and Tc2= 1.72K. They are used to draw a precise magnetic field-temperature superconducting phase diagram of PrOs4Sb12 down to 350 mK. We discuss the superconducting phase diagram of PrOs4Sb12 and its possible relation with an unconventional superconducting order parameter. We give a detailed analysis of Hc2(T), which shows paramagnetic limitation (a support for even parity pairing) and multiband effects.

cond-mat.supr-con↗

arXiv2003

Phase diagram of heavy fermion systems

The Meccano of heavy fermion systems is shown on different cases going from anomalous monochalcogenides to cerium intermetallic compounds with special focus on the ideal case of the CeRu2Si2 series. Discussion is made in the frame of the interplay between valence, electronic structure (Fermi surface), and magnetism. The nice tools given by the temperature, the pressure, and the magnetic field allow to explore different ground states as well as the slow downhill ''race'' before reaching a Fermi liquid finish line at very low temperature. Experimentally, the Gr"uneisen parameter i.e. the ratio of the thermal expansion by the specific heat is a coloured magic number; its temperature, pressure, and magnetic field dependence is a deep disclosure of competing hierarchies and the conversion of this adaptive matter to external responses.

cond-mat.str-el↗

arXiv2002

Phonon anomalies at the valence transition of SmS : An inelasticX-ray scattering study under pressure

The phonon dispersion curve of SmS under pressure was studied by inelastic x-ray scattering around the pressure-induced valence transition. A significant softening of the longitudinal acoustic modes propagating along the [111] direction was observed spanning a wide $q$ region from ($\frac{2π}{3a},\frac{2π}{3a},\frac{2π}{3a}$) up to the zone boundary as SmS becomes metallic. The largest softening occurs at the zone boundary and stays stable up to the highest measured pressure of 80 kbar while a gradual hardening of the low $q$ modes simultaneously appears. This phonon spectrum indicates favorable conditions for the emergence of pressure-induced superconductivity in SmS.

cond-mat.str-el↗

arXiv1998

Step Structure Observed in the Superconducting Transition of a Bulk Pb Wire with Very Low Current Densities

We present measurements obtained on a bulk cylinder of lead with a SQUID picovoltmeter. A distinctive hysteretic step structure is observed. It is washed out on increasing the current and reinforced in higher field. Classical models for the dissipation mechanisms in the intermediate state are briefly reviewed and used to give a first interpretation of these results, although a complete understanding is not reached yet.

cond-mat.supr-con↗