Steffen Wirth

Hall-effect evolution across a heavy-fermion quantum critical point

A quantum critical point (QCP) develops in a material at absolute zero when a new form of order smoothly emerges in its ground state. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs. To accommodate the heavy electrons, the Fermi surface of the heavy-fermion paramagnet is larger than that of an antiferromagnet. An important unsolved question is whether the Fermi surface transformation at the QCP develops gradually, as expected if the magnetism is of spin-density-wave (SDW) type, or suddenly, as expected if the heavy electrons are abruptly localized by magnetism. Here we report measurements of the low-temperature Hall coefficient (RH)—a measure of the Fermi surface volume—in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. RH undergoes an increasingly rapid change near the QCP as the temperature is lowered, extrapolating to a sudden jump in the zero temperature limit. We interpret these results in terms of a collapse of the large Fermi surface and of the heavy-fermion state itself precisely at the QCP.

Noise probe of the dynamic phase separation in La(2/3)Ca(1/3)MnO3

Giant random telegraph noise (RTN) in the resistance fluctuation of a macroscopic film of perovskite-type manganese oxide La(2/3)Ca(1/3)MnO3 has been observed at various temperatures ranging from 4 to 170 K, well below the Curie temperature ( T(C) approximately 210 K). The amplitudes of the two-level fluctuations vary from 0.01% to 0.2%. We discuss the origin of the RTN to be a dynamic mixed-phase percolative conduction process, where manganese clusters switch back and forth between two phases that differ in their conductivity and magnetization.

Hall magnetometry on a single iron nanoparticle

High-sensitivity magnetometry over a wide temperature range has been achieved using submicron GaAs/GaAlAs Hall gradiometry. The sensitivity and versatility of the technique was demonstrated by the successful measurement of the magnetization switching of a single Fe nanoparticle with m∼5×105 μB (∼5×10−15 emu) at temperatures as high as 75 K.

Fermi-surface collapse and dynamical scaling near a quantum-critical point

Quantum criticality arises when a macroscopic phase of matter undergoes a continuous transformation at zero temperature. While the collective fluctuations at quantum-critical points are being increasingly recognized as playing an important role in a wide range of quantum materials, the nature of the underlying quantum-critical excitations remains poorly understood. Here we report in-depth measurements of the Hall effect in the heavy-fermion metal YbRh2Si2, a prototypical system for quantum criticality. We isolate a rapid crossover of the isothermal Hall coefficient clearly connected to the quantum-critical point from a smooth background contribution; the latter exists away from the quantum-critical point and is detectable through our studies only over a wide range of magnetic field. Importantly, the width of the critical crossover is proportional to temperature, which violates the predictions of conventional theory and is instead consistent with an energy over temperature, E/T, scaling of the quantum-critical single-electron fluctuation spectrum. Our results provide evidence that the quantum-dynamical scaling and a critical Kondo breakdown simultaneously operate in the same material. Correspondingly, we infer that macroscopic scale-invariant fluctuations emerge from the microscopic many-body excitations associated with a collapsing Fermi-surface. This insight is expected to be relevant to the unconventional finite-temperature behavior in a broad range of strongly correlated quantum systems.

Gas phase interstitial modification of rare-earth intermetallics

The gas-phase interstitial modification of rare-earth intermetallics is studied. Net reaction energies for nitrogen in Sm/sub 2/Fe/sub 17/ and Nd(Fe/sub 11/Tl) are U/sub 0/=-57 kJ/mole and U/sub 0/=-51 kJ/mole, respectively. The equilibrium nitrogen concentration is calculated as function of temperature and gas pressure using a simple lattice gas model. For nitrogen in Sm/sub 2/Fe/sub 17/, refined diffusion parameters D/sub 0/(N)=1.02 mm/sup 2//s and E/sub a/(N)=133 kJ/mole, determined by thermoplexic analysis of the initial stage of nitrogen absorption, are used to calculate nitrogen profiles and the time dependence of the mean nitrogen content during nitrogenation. Similar values are obtained for nitrogen in Nd(Fe/sub 11/Tl), whereas the activation energies for hydrogen in Sm/sub 2/Fe/sub 17/ and Nd(Fe/sub 11/Tl) are 31 kJ/mole and 45 kJ/mole, respectively. The elastic stress and strain profiles during nitrogenation are calculated. Important results are a large uniaxial strain near the surface of nonuniformly nitrided particles, and core expansion even in the absence of any nitrogen there. >

Thermal and electrical transport across a magnetic quantum critical point

A quantum critical point (QCP) arises when a continuous transition between competing phases occurs at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau’s Fermi-liquid description, which is the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations that possess the quantum numbers of the non-interacting electrons. Here we report measurements of thermal and electrical transport across the field-induced magnetic QCP in the heavy-fermion compound YbRh2Si2 (refs 2, 3). We show that the ratio of the thermal to electrical conductivities at the zero-temperature limit obeys the Wiedemann–Franz law for magnetic fields above the critical field at which the QCP is attained. This is also expected for magnetic fields below the critical field, where weak antiferromagnetic order and a Fermi-liquid phase form below 0.07 K (at zero field). At the critical field, however, the low-temperature electrical conductivity exceeds the thermal conductivity by about 10 per cent, suggestive of a non-Fermi-liquid ground state. This apparent violation of the Wiedemann–Franz law provides evidence for an unconventional type of QCP at which the fundamental concept of Landau quasiparticles no longer holds. These results imply that Landau quasiparticles break up, and that the origin of this disintegration is inelastic scattering associated with electronic quantum critical fluctuations—these insights could be relevant to understanding other deviations from Fermi-liquid behaviour frequently observed in various classes of correlated materials.

Hybridization gap and Fano resonance in SmB6

Significance Quantum entanglement may give rise to emerging phenomena and new states of matter. In the intermediate-valence material SmB6, this is realized via hybridization between localized 4f and conduction band states that, at sufficiently low temperatures, results in the screening of the 4f local moments by the conduction electrons and produces a so-called Kondo resonance in the density of states. The latter is examined by scanning tunneling spectroscopy down to temperatures well below the Kondo temperature. This atomically resolved spectroscopy allows one to distinguish between reconstructed and pristine surfaces and different surface terminations, all of which influence the spectroscopic results. These insights are vital for other, less local spectroscopic tools in the context of possible topologically protected surface states. Hybridization between conduction electrons and the strongly interacting f-electrons in rare earth or actinide compounds may result in new states of matter. Depending on the exact location of the concomitant hybridization gap with respect to the Fermi energy, a heavy fermion or an insulating ground state ensues. To study this entanglement locally, we conducted scanning tunneling microscopy and spectroscopy (STS) measurements on the “Kondo insulator” SmB6. The vast majority of surface areas investigated were reconstructed, but infrequently, patches of varying sizes of nonreconstructed Sm- or B-terminated surfaces also were found. On the smallest patches, clear indications for the hybridization gap with logarithmic temperature dependence (as expected for a Kondo system) and for intermultiplet transitions were observed. On nonreconstructed surface areas large enough for coherent cotunneling, we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure, not proving but leaving open the possibility of the existence of a topologically protected surface state.

Structural and magnetic properties of Ni2MnGa

Abstract The martensitic phase transition from a cubic to a tetragonal structure on cooling in the shape-memory ferromagnetic Heusler alloy Ni2MnGa is investigated. The martensitic-transition temperature TM, known to be 205 K for the nominal composition, was expected to increase with increasing Ni content of the alloy. For the samples Ni2(1 + δ)Mn1 − δGa1 − δ with δ = 0, 0.046 and 0.10, respectively, the Curie temperature has been determined as 380 K independent of composition. TM for the sample with δ = 0.046 is also near 205 K, but the sample with δ = 0.10 is single phase and remains tetragonal up to the Curie temperature. The spontaneous polarization of this magnetically uniaxial phase has been determined to be JS = 0.16T and an estimate of the magnetocrystalline anisotropy constant is K1 ≈ 80kJm−3.

Magnetic interactions in nanometer-scale particle arrays grown onto permalloy films

The magnetic interactions in arrays of nanometer-scale ferromagnetic iron particles enhanced by direct growth onto thin permalloy films were investigated. The magnetic measurements [Hall magnetometry up to 100 K and variable field MFM (magnetic force microscopy) at room temperature] showed that the magnetization behavior of the permalloy was strongly influenced by the presence of the small (∼13 nm in diameter) particles. The mean values of the particles’ switching fields coincided with those for noninteracting particles. The switching field distribution of the iron particles, however, was considerably broadened by their interactions. These results for strongly interacting small particles exemplify the magnetization behavior of ever smaller and more dense magnetic storage media.

Correlation between ground state and orbital anisotropy in heavy fermion materials

Significance The ground state of materials with strong electronic correlations depends on a delicate balance among competing interactions. The strongly correlated compounds CeMIn5, with M = Co, Rh, and Ir, exhibit superconducting and magnetic ground states as well as Fermi surface changes upon substituting one M element for another and become even higher temperature superconductors when Ce is substituted by Pu. They are therefore recognized as important model systems in which a search for parameters correlating with the occurrence of these ground states could be successful. The present X-ray absorption study of CeRh1−xIrxIn5 reveals that anisotropy of the Ce 4f-wave function is a significant parameter that is highly sensitive to the ground-state formation and should be taken into account when modeling these systems. The interplay of structural, orbital, charge, and spin degrees of freedom is at the heart of many emergent phenomena, including superconductivity. Unraveling the underlying forces of such novel phases is a great challenge because it not only requires understanding each of these degrees of freedom, it also involves accounting for the interplay between them. Cerium-based heavy fermion compounds are an ideal playground for investigating these interdependencies, and we present evidence for a correlation between orbital anisotropy and the ground states in a representative family of materials. We have measured the 4f crystal-electric field ground-state wave functions of the strongly correlated materials CeRh1−xIrxIn5 with great accuracy using linear polarization-dependent soft X-ray absorption spectroscopy. These measurements show that these wave functions correlate with the ground-state properties of the substitution series, which covers long-range antiferromagnetic order, unconventional superconductivity, and coexistence of these two states.