Superconductivity in an extreme strange metal.

Some of the highest-transition-temperature superconductors across various materials classes exhibit linear-in-temperature 'strange metal' or 'Planckian' electrical resistivities in their normal state. It is thus believed by many that this behavior holds the key to unlock the secrets of high-temperature superconductivity. However, these materials typically display complex phase diagrams governed by various competing energy scales, making an unambiguous identification of the physics at play difficult. Here we use electrical resistivity measurements into the micro-Kelvin regime to discover superconductivity condensing out of an extreme strange metal state-with linear resistivity over 3.5 orders of magnitude in temperature. We propose that the Cooper pairing is mediated by the modes associated with a recently evidenced dynamical charge localization-delocalization transition, a mechanism that may well be pertinent also in other strange metal superconductors.

105Pd NMR and NQR study of the cubic heavy fermion system Ce3Pd20Si6.

We report 105Pd nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) measurements on a single crystal of Ce3Pd20Si6, where antiferroquadrupolar and antiferromagnetic orders develop at low temperature. From the analysis of NQR and NMR spectra, we have determined the electric field gradient (EFG) tensors and the anisotropic Knight shift (K) components for both inequivalent Pd sites-Pd(32f) and Pd(48h). The observed EFG values are in excellent agreement with our state-of-the-art density functional theory calculations. The principal values of the quadrupolar coupling are [Formula: see text] MHz and [Formula: see text] MHz, for the Pd(32f) and Pd(48h) sites, respectively, which is large compared to the Larmor frequency defined by the gyromagnetic constant [Formula: see text] MHz/T for 105Pd. Therefore, the complete knowledge of K and the EFG tensors is crucial to establish the correspondence between NMR spectra and crystallographic sites, which is needed for a complete analysis of the magnetic structure, static spin susceptibility, and the spin-lattice relaxation rate data and a better understanding of the groundstate of Ce3Pd20Si6.

Singular charge fluctuations at a magnetic quantum critical point.

Strange metal behavior is ubiquitous in correlated materials, ranging from cuprate superconductors to bilayer graphene, and may arise from physics beyond the quantum fluctuations of a Landau order parameter. In quantum-critical heavy-fermion antiferromagnets, such physics may be realized as critical Kondo entanglement of spin and charge and probed with optical conductivity. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy-grown thin films of YbRh2Si2, a model strange-metal compound. We observed frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery suggests that critical charge fluctuations play a central role in the strange metal behavior, elucidating one of the long-standing mysteries of correlated quantum matter.

Adverse events associated with deep brain stimulation in patients with childhood-onset dystonia.

BACKGROUND: Data on pediatric DBS is still limited because of small numbers in single center series and lack of systematic multi-center trials. OBJECTIVES: We evaluate short- and long-term adverse events (AEs) of patients undergoing deep brain stimulation (DBS) during childhood and adolescence. METHODS: Data collected by the German registry on pediatric DBS (GEPESTIM) were analyzed according to reversible and irreversible AEs and time of occurrence with relation to DBS-surgery: Intraoperative, perioperative (<4 weeks), postoperative (4 weeks < 6 months) and long term AEs (>6 months). RESULTS: 72 patients with childhood-onset dystonia from 10 DBS-centers, who received 173 DBS electrodes and 141 implantable pulse generators (IPG), were included in the registry. Mean time of postoperative follow-up was 4.6 ±â€¯4 years. In total, 184 AEs were documented in 53 patients (73.6%). 52 DBS-related AEs in 26 patients (36.1%) required 45 subsequent surgical interventions 4.7 ±â€¯4.1 years (range 3 months-15 years) after initial implantation. The total risk of an AE requiring surgical intervention was 7.9% per electrode-year. Hardware-related AEs were the most common reason for surgery. There was a tendency of a higher rate of AEs in patients aged 7-9 years beyond 6 months after implantation. DISCUSSION: The intraoperative risk of AEs in pediatric patients with dystonia undergoing DBS is very low, whereas the rate of postoperative hardware-related AEs is a prominent feature with a higher occurrence compared to adults, especially on long-term follow-up. CONCLUSION: Factors leading to such AEs must be identified and patient management has to be focused on risk minimization strategies in order to improve DBS therapy and maximize outcome in pediatric patients.

Kondo-like phonon scattering in thermoelectric clathrates.

Crystalline solids are generally known as excellent heat conductors, amorphous materials or glasses as thermal insulators. It has thus come as a surprise that certain crystal structures defy this paradigm. A prominent example are type-I clathrates and other materials with guest-host structures. They sustain low-energy Einstein-like modes in their phonon spectra, but are also prone to various types of disorder and phonon-electron scattering and thus the mechanism responsible for their ultralow thermal conductivities has remained elusive. Our thermodynamic and transport measurements on various clathrate single crystal series and their comparison with ab initio simulations reveal an all phononic Kondo effect as origin. This insight devises design strategies to further suppress the thermal conductivity of clathrates and other related materials classes, with relevance for thermoelectric waste heat recovery and, more generally, phononic applications. It may also trigger theoretical work on strong correlation effects in phonon systems.

Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling.

Recent theoretical studies of topologically nontrivial electronic states in Kondo insulators have pointed to the importance of spin-orbit coupling (SOC) for stabilizing these states. However, systematic experimental studies that tune the SOC parameter λ_{SOC} in Kondo insulators remain elusive. The main reason is that variations of (chemical) pressure or doping strongly influence the Kondo coupling J_{K} and the chemical potential µ-both essential parameters determining the ground state of the material-and thus possible λ_{SOC} tuning effects have remained unnoticed. Here, we present the successful growth of the substitution series Ce_{3}Bi_{4}(Pt_{1-x}Pd_{x})_{3} (0≤x≤1) of the archetypal (noncentrosymmetric) Kondo insulator Ce_{3}Bi_{4}Pt_{3}. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and it therefore is likely to leave J_{K} and µ essentially unchanged. By contrast, the large mass difference between the 5d element Pt and the 4d element Pd leads to a large difference in λ_{SOC}, which thus is the dominating tuning parameter in the series. Surprisingly, with increasing x (decreasing λ_{SOC}), we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce_{3}Bi_{4}Pd_{3} shows thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.

Localization of propagative phonons in a perfectly crystalline solid.

Perfectly crystalline solids are excellent heat conductors. Prominent counterexamples are intermetallic clathrates, guest-host systems with a high potential for thermoelectric applications due to their ultralow thermal conductivities. Our combined experimental and theoretical investigation of the lattice dynamics of a particularly simple binary representative, Ba(8)Si(46), identifies the mechanism responsible for the reduction of lattice thermal conductivity intrinsic to the perfect crystal structure. Above a critical wave vector, the purely harmonic guest-host interaction leads to a drastic transfer of spectral weight to the guest atoms, corresponding to a localization of the propagative phonons.

Thermopower enhancement by encapsulating cerium in clathrate cages.

The increasing worldwide energy consumption calls for the design of more efficient energy systems. Thermoelectrics could be used to convert waste heat back to useful electric energy if only more efficient materials were available. The ideal thermoelectric material combines high electrical conductivity and thermopower with low thermal conductivity. In this regard, the intermetallic type-I clathrates show promise with their exceedingly low lattice thermal conductivities. Here we report the successful incorporation of cerium as a guest atom into the clathrate crystal structure. In many simpler intermetallic compounds, this rare earth element is known to lead, through the Kondo interaction, to strong correlation phenomena including the occurrence of giant thermopowers at low temperatures. Indeed, we observe a 50% enhancement of the thermopower compared with a rare-earth-free reference material. Importantly, this enhancement occurs at high temperatures and we suggest that a rattling-enhanced Kondo interaction underlies this effect.

Neurostimulation for Parkinson's disease with early motor complications.

BACKGROUND: Subthalamic stimulation reduces motor disability and improves quality of life in patients with advanced Parkinson's disease who have severe levodopa-induced motor complications. We hypothesized that neurostimulation would be beneficial at an earlier stage of Parkinson's disease. METHODS: In this 2-year trial, we randomly assigned 251 patients with Parkinson's disease and early motor complications (mean age, 52 years; mean duration of disease, 7.5 years) to undergo neurostimulation plus medical therapy or medical therapy alone. The primary end point was quality of life, as assessed with the use of the Parkinson's Disease Questionnaire (PDQ-39) summary index (with scores ranging from 0 to 100 and higher scores indicating worse function). Major secondary outcomes included parkinsonian motor disability, activities of daily living, levodopa-induced motor complications (as assessed with the use of the Unified Parkinson's Disease Rating Scale, parts III, II, and IV, respectively), and time with good mobility and no dyskinesia. RESULTS: For the primary outcome of quality of life, the mean score for the neurostimulation group improved by 7.8 points, and that for the medical-therapy group worsened by 0.2 points (between-group difference in mean change from baseline to 2 years, 8.0 points; P=0.002). Neurostimulation was superior to medical therapy with respect to motor disability (P<0.001), activities of daily living (P<0.001), levodopa-induced motor complications (P<0.001), and time with good mobility and no dyskinesia (P=0.01). Serious adverse events occurred in 54.8% of the patients in the neurostimulation group and in 44.1% of those in the medical-therapy group. Serious adverse events related to surgical implantation or the neurostimulation device occurred in 17.7% of patients. An expert panel confirmed that medical therapy was consistent with practice guidelines for 96.8% of the patients in the neurostimulation group and for 94.5% of those in the medical-therapy group. CONCLUSIONS: Subthalamic stimulation was superior to medical therapy in patients with Parkinson's disease and early motor complications. (Funded by the German Ministry of Research and others; EARLYSTIM ClinicalTrials.gov number, NCT00354133.).

BH3-only proteins are tail-anchored in the outer mitochondrial membrane and can initiate the activation of Bax.

During mitochondrial apoptosis, pro-apoptotic BH3-only proteins cause the translocation of cytosolic Bcl-2-associated X protein (Bax) to the outer mitochondrial membrane (OMM) where it is activated to release cytochrome c from the mitochondrial intermembrane space, but the mechanism is under dispute. We show that most BH3-only proteins are mitochondrial proteins that are imported into the OMM via a C-terminal tail-anchor domain in isolated yeast mitochondria, independently of binding to anti-apoptotic Bcl-2 proteins. This C-terminal domain acted as a classical mitochondrial targeting signal and was sufficient to direct green fluorescent protein to mitochondria in human cells. When expressed in mouse fibroblasts, these BH3-only proteins localised to mitochondria and were inserted in the OMM. The BH3-only proteins Bcl-2-interacting mediator of cell death (Bim), tBid and p53-upregulated modulator of apoptosis sensitised isolated mitochondria from Bax/Bcl-2 homologous antagonist/killer-deficient fibroblasts to cytochrome c-release by recombinant, extramitochondrial Bax. For Bim, this activity is shown to require the C-terminal-targeting signal and to be independent of binding capacity to and presence of anti-apoptotic Bcl-2 proteins. Bim further enhanced Bax-dependent killing in yeast. A model is proposed where OMM-tail-anchored BH3-only proteins permit passive 'recruitment' and catalysis-like activation of extra-mitochondrial Bax. The recognition of C-terminal membrane-insertion of BH3-only proteins will permit the development of a more detailed concept of the initiation of mitochondrial apoptosis.

Destruction of the Kondo effect in the cubic heavy-fermion compound Ce3Pd20Si6.

How ground states of quantum matter transform between one another reveals deep insights into the mechanisms stabilizing them. Correspondingly, quantum phase transitions are explored in numerous materials classes, with heavy-fermion compounds being among the most prominent ones. Recent studies in an anisotropic heavy-fermion compound have shown that different types of transitions are induced by variations of chemical or external pressure, raising the question of the extent to which heavy-fermion quantum criticality is universal. To make progress, it is essential to broaden both the materials basis and the microscopic parameter variety. Here, we identify a cubic heavy-fermion material as exhibiting a field-induced quantum phase transition, and show how the material can be used to explore one extreme of the dimensionality axis. The transition between two different ordered phases is accompanied by an abrupt change of Fermi surface, reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in the anisotropic YbRh2Si2. This finding leads to a materials-based global phase diagram--a precondition for a unified theoretical description.

Locating the STN-DBS electrodes and resolving their subsequent networks using coherent source analysis on EEG.

The deep brain stimulation (DBS) of the subthalamic nucleus (STN) is the most effective surgical therapy for Parkinson's disease (PD). The first aim of the study was to locate the STN-DBS electrode by applying source analysis on EEG. Secondly, to identify tremor related areas which are associated with the STN. The Dynamic imaging of coherent sources (DICS) was used to find the coherent sources in the brain. The capability of the source analysis to detect deep sources like STN in the brain using EEG data was tested with two model dipole simulations. The simulations were concentrated on two aspects, the angle of the dipole orientation and the disturbance of the cortical areas on locating subcortical regions. In all the DBS treated Parkinsonian tremor patients the power spectrum showed a clear peak at the stimulated frequency and followed by there harmonics. The DBS stimulated frequency constituted a network of primary sensory motor cortex, supplementary motor area, prefrontal cortex, diencephalon, cerebellum and brainstem. Thus the STN was located in the region of the diencephalon. The resolved network may give better understanding to the pathophysiology of the effected tremor network in PD patients with STN-DBS.

Chemical pressure, dilution and disorder in the heavy fermion compounds Ce(3-x)La(x)Pd20Si6 (x=1/3, 2/3).

The heavy fermion compound Ce3Pd20Si6 is one of the rare examples of a cubic system with a readily accessible quantum critical point. Ce atoms at the two different sites 4a and 8c of the crystal structure have recently been shown to have different crystal field ground states (Γ7 and Γ8, respectively) and are assumed to be responsible for the two different low-temperature phase transitions at T(L) and T(U), which have been tentatively attributed to antiferromagnetic and antiferroquadrupolar order, respectively. Here we present electrical resistivity measurements in a wide temperature range (50 mK-300 K) on two new representatives of the La substitution series Ce(3 - x)La(x)Pd20Si6, x=1/3 and 2/3. Put in the context of previously published data our results indicate that La preferentially occupies the 4a site and that Ce ions at the 8c site have a sizably larger Kondo temperature. Low-temperature resistivity measurements in applied magnetic fields suggest that (disorder smeared) quantum critical points exist in the x=1/3 and 2/3 samples at fields below 1 T.

Evidence for a non-Fermi-liquid phase in Ge-substituted YbRh2Si2.

The canonical view of heavy fermion quantum criticality assumes a single quantum critical point separating the paramagnet from the antiferromagnet. However, recent experiments on Yb-based heavy fermion compounds suggest the presence of non-Fermi liquid behavior over a finite zero-temperature region. Using detailed susceptibility and transport measurements we show that the classic quantum critical system, Ge-substituted YbRh(2)Si(2), also displays such behavior. We advance arguments that this is not due to a disorder-smeared quantum critical point, but represents a new class of metallic phase.

Atomic ordering and thermoelectric properties of the n-type clathrate Ba8Ni3.5Ge42.1square0.4.

Single crystals of the type-I clathrate Ba(8)Ni(3.5)Ge(42.1)square(0.4) (space group Pm3n, no. 223, a = 10.798(2) A, l = 30 mm, slashed circle = 8 mm) were grown from the melt using the Bridgman technique. Their composition, determined by microprobe analysis, reveals a distinctly lower Ni content than previously reported for the lower limit (x = 5.4) of the homogeneity range of the clathrate-I phase Ba(8)Ni(x)Ge(46-x). From single crystal X-ray diffraction data we introduce a crystal structure model that takes point defects (vacancies) square in the Ge network into account. It reveals that both Ni and square accumulate at a single site (6c) and that, as a consequence, the Ge network distorts considerably. Ba(8)Ni(3.5)Ge(42.1)square(0.4) shows metal-like behaviour (drho/dT > 0) albeit with high resistivity at room temperature (rho(300 K) approximately 1 mOmega cm). Together with the low charge carrier concentration of 2.3 e(-)/unit cell at 300 K this is typical of a degenerate semiconductor. The lattice thermal conductivity is distinctly smaller than that of Ba(8)Ge(43)square(3), where the vacancies partially order, and smaller than those of Ba-Ni-Ge type-I clathrates without vacancies, suggesting that disordered vacancies efficiently scatter heat-transporting phonons. We provide evidence that the maximum value of the thermoelectric figure of merit reached in Ba(8)Ni(3.5)Ge(42.1)square(0.4), ZT(680 K) congruent with 0.21, can be further improved by adjusting the charge carrier concentration.

Multiple energy scales at a quantum critical point.

We report thermodynamic measurements in a magnetic-field-driven quantum critical point of a heavy fermion metal, YbRh2Si2. The data provide evidence for an energy scale in the equilibrium excitation spectrum that is in addition to the one expected from the slow fluctuations of the order parameter. Both energy scales approach zero as the quantum critical point is reached, thereby providing evidence for a new class of quantum criticality.

Two-channel Kondo effect in glasslike ThAsSe.

We present low-temperature heat and charge transport as well as caloric properties of a ThAsSe single crystal. An extra -AT(1/2) term in the electrical resistivity, independent of magnetic fields as high as 14 T, provides evidence for an unusual scattering of conduction electrons. Additionally, both the thermal conductivity and the specific heat show a glass-type temperature dependence which signifies the presence of tunneling states. These observations apparently point to an experimental realization of a two-channel Kondo effect derived from structural two-level systems.

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 (R(H))--a measure of the Fermi surface volume--in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. R(H) 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.