H. Wilhelm

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 the heavy-electron material YbRh2Si2 found that near a field-induced QCP 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 scattering cross-section. But these studies could not shed light on whether these properties were an artefact of the applied field, or a more general feature of field-free QCPs. Here we report that, when germanium-doped YbRh2Si2 is tuned away from a chemically induced QCP by magnetic fields, there is a universal behaviour 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.

Correlated defect nanoregions in a metal–organic framework

Throughout much of condensed matter science, correlated disorder is key to material function. While structural and compositional defects are known to exist within a variety of metal–organic frameworks, the prevailing understanding is that these defects are only ever included in a random manner. Here we show—using a combination of diffuse scattering, electron microscopy, anomalous X-ray scattering, and pair distribution function measurements—that correlations between defects can in fact be introduced and controlled within a hafnium terephthalate metal–organic framework. The nanoscale defect structures that emerge are an analogue of correlated Schottky vacancies in rocksalt-structured transition metal monoxides and have implications for storage, transport, optical and mechanical responses. Our results suggest how the diffraction behaviour of some metal–organic frameworks might be reinterpreted, and establish a strategy of exploiting correlated nanoscale disorder as a targetable and desirable motif in metal–organic framework design.

Structure of sodium above 100 GPa by single-crystal x-ray diffraction

At pressures above a megabar (100 GPa), sodium crystallizes in a number of complex crystal structures with unusually low melting temperatures, reaching as low as 300 K at 118 GPa. We have utilized this unique behavior at extreme pressures to grow a single crystal of sodium at 108 GPa, and have investigated the complex crystal structure at this pressure using high-intensity x-rays from the new Diamond synchrotron source, in combination with a pressure cell with wide angular apertures. We confirm that, at 108 GPa, sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. The results demonstrate the extension of single-crystal structure refinement beyond 100 GPa and raise the prospect of successfully determining the structures of yet more complex phases reported in sodium and other elements at extreme pressures.

From spin-Peierls to superconductivity: (TMTTF)2PF6 under high pressure

The nature of the attractive electron-electron interaction, leading to the formation of Cooper pairs in unconventional superconductors, has still to be fully understood and is subject to intensive research. Here we show that the sequence spin-Peierls, antiferromagnetism, superconductivity observed in (TMTTF)2PF6 under pressure makes the (TM)2X phase diagram universal. We argue that the suppression of the spin-Peierls transition under pressure, the close vicinity of antiferromagnetic and superconducting phases at high pressure, as well as the existence of critical antiferromagnetic fluctuations above Tc strongly support the intriguing possibility that the interchain exchange of antiferromagnetic fluctuations provides the pairing mechanism required for bound charge carriers.

A compensated heat-pulse calorimeter for low temperatures

We describe a technique for measuring heat capacities (Cmin≈1 μJ/K at 0.1 K) of small solid samples at low temperatures (0.03 K<T<6 K) and in high magnetic fields (B<12 T). In this compensated heat-pulse technique the thermal losses are compensated through a background heating. A detailed analysis of the heat flow takes the heat input and losses into account. Test measurements on tin and YbRh2(Si0.95Ge0.05)2 showed that the heat capacity can be determined with high precision in a fast and accurate way. This technique provides a versatile calorimeter for a wide range of heat capacities which achieves its main performance if several sample platforms are mounted and one sample is measured while the other may cool down.

The case for universality of the phase diagram of the Fabre and Bechgaard salts

Abstract:We report the observation of superconductivity in the spin-Peierls Fabre salt (TMTTF)2PF6 from pressure dependent electrical transport measurements above a threshold of 4.35 GPa. The data complete the sequence of ground states of this compound in the temperature and pressure plane adducing an empirical basis to the universal character of the phase diagram of the Fabre salts and their selenide analogues, the Bechgaard salts. The structure of the phase diagram at the approach of the crossover between spin-density wave and superconducting states is compared with the results of scaling theory of the interplay between both electronic instabilities under pressure. The comparison supports the view that magnetism and superconductivity in these compounds have a common electronic origin.

Transport evidence for pressure-induced superconductivity in CePd2Si2

Resistivity measurements performed under pressure on high quality single crystals of CePd2Si2 are reported. Pressure-induced superconductivity is observed in the range 2–7 GPa with an optimal Tc of 520 mK at 5.1 GPa. The superconducting properties of the sample as well as its peculiar zero pressure resistivity are tentatively linked to its preparation.

Metallic state in cubic FeGe beyond its quantum phase transition

We report on results of electrical resistivity and structural investigations on the cubic modification of FeGe under high pressure. The long-wavelength helical order (T(C) = 280 K) is suppressed at a critical pressure p(c) approximately 19 GPa. An anomaly at T(X)(p) and strong deviations from a Fermi-liquid behavior in a wide pressure range above p(c) suggest that the suppression of T(C) disagrees with the standard notion of a quantum critical phase transition. The metallic ground state persisting at high pressure can be described by band-structure calculations if zero-point motion is included. The shortest FeGe interatomic distance display discontinuous changes in the pressure dependence close to the T(C)(p) phase line.

Transport properties of Yb-compounds at high pressure

Abstract We report investigations under pressure ( P ) of the resistivity ( ρ ) and the thermopower ( S ) of YbInAu 2 , YbCu 2 Si 2 , YbCuAl and YbSi. Magnetic ordering can be induced by P in YbCu 2 Si 2 , like in YbCuAl, whereas for YbSi a gap at low temperature opens. The crystal field effect could be responsible for the sign change of S at low temperature.

Scaling study and thermodynamic properties of the cubic helimagnet FeGe

The critical behavior of the cubic helimagnet FeGe was obtained from isothermal magnetization data in very close vicinity of the ordering temperature. A thorough and consistent scaling analysis of these data revealed the critical exponents