Phase stabilization by electronic entropy in plutonium

Plutonium metal undergoes an anomalously large 25% collapse in volume from its largest volume δ phase (δ-Pu) to its low temperature α phase, yet the underlying thermodynamic mechanism has largely remained a mystery. Here we use magnetostriction measurements to isolate a previously hidden yet substantial electronic contribution to the entropy of δ-Pu, which we show to be crucial for the stabilization of this phase. The entropy originates from two competing instabilities of the 5f-electron shell, which we show to drive the volume of Pu in opposing directions, depending on the temperature and volume. Using calorimetry measurements, we establish a robust thermodynamic connection between the two excitation energies, the atomic volume, and the previously reported excess entropy of δ-Pu at elevated temperatures.

Plutonium has strong electronic correlations, which makes it difficult to establish microscopic understanding of the complicated structural phase diagram. Here the authors identify an electronic contribution to the entropy that stabilises the δ-Pu phase.

Resonant torsion magnetometry in anisotropic quantum materials

Unusual behavior in quantum materials commonly arises from their effective low-dimensional physics, reflecting the underlying anisotropy in the spin and charge degrees of freedom. Here we introduce the magnetotropic coefficient k = ∂²F/∂θ², the second derivative of the free energy F with respect to the magnetic field orientation θ in the crystal. We show that the magnetotropic coefficient can be quantitatively determined from a shift in the resonant frequency of a commercially available atomic force microscopy cantilever under magnetic field. This detection method enables part per 100 million sensitivity and the ability to measure magnetic anisotropy in nanogram-scale samples, as demonstrated on the Weyl semimetal NbP. Measurement of the magnetotropic coefficient in the spin-liquid candidate RuCl₃ highlights its sensitivity to anisotropic phase transitions and allows a quantitative comparison to other thermodynamic coefficients via the Ehrenfest relations.

Scale-invariant magnetoresistance in a cuprate superconductor

The anomalous metallic state in the high-temperature superconducting cuprates is masked by superconductivity near a quantum critical point. Applying high magnetic fields to suppress superconductivity has enabled detailed studies of the normal state, yet the direct effect of strong magnetic fields on the metallic state is poorly understood. We report the high-field magnetoresistance of thin-film La_2-x Sr _x CuO₄ cuprate in the vicinity of the critical doping, 0.161 ≤ p ≤ 0.190. We find that the metallic state exposed by suppressing superconductivity is characterized by magnetoresistance that is linear in magnetic fields up to 80 tesla. The magnitude of the linear-in-field resistivity mirrors the magnitude and doping evolution of the well-known linear-in-temperature resistivity that has been associated with quantum criticality in high-temperature superconductors.

Robust spin correlations at high magnetic fields in the harmonic honeycomb iridates

The complex antiferromagnetic orders observed in the honeycomb iridates are a double-edged sword in the search for a quantum spin-liquid: both attesting that the magnetic interactions provide many of the necessary ingredients, while simultaneously impeding access. Focus has naturally been drawn to the unusual magnetic orders that hint at the underlying spin correlations. However, the study of any particular broken symmetry state generally provides little clue about the possibility of other nearby ground states. Here we use magnetic fields approaching 100 Tesla to reveal the extent of the spin correlations in γ-lithium iridate. We find that a small component of field along the magnetic easy-axis melts long-range order, revealing a bistable, strongly correlated spin state. Far from the usual destruction of antiferromagnetism via spin polarization, the high-field state possesses only a small fraction of the total iridium moment, without evidence for long-range order up to the highest attainable magnetic fields.The complex antiferromagnetic orders observed in the honeycomb iridates prevent access to a spin-liquid ground state. Here the authors apply extremely high magnetic fields to destroy the antiferromagnetic order in γ-lithium iridate and reveal a bistable, strongly correlated spin state.

Cascade of magnetic field induced spin transitions in LaCoO3

We present magnetization and magnetostriction studies of LaCoO3 in magnetic fields approaching 100 T. In contrast with expectations from single-ion models, the data reveal two distinct first-order transitions and well-defined magnetization plateaus. The magnetization at the higher plateau is only about half the saturation value expected for spin-1 Co3+ ions. These findings strongly suggest collective behavior induced by interactions between different electronic configurations of Co3+ ions. We propose a model that predicts crystalline spin textures and a cascade of four magnetic phase transitions at high fields, of which the first two account for the experimental data.

Observation of a martensitic structural distortion in V, Nb, and Ta

Thermal-expansion measurements of the Group 5 elements V, Nb, and Ta reveal a structural distortion below 300 K. Data for single-crystalline Nb and Ta display anisotropic thermal expansion, martensitic in character, that is inconsistent with cubic crystal structures at low temperature. Published results on V show similar behavior. Interstitial impurities suppress the transition.

Polycrystalline gamma-plutonium's elastic moduli versus temperature

Resonant ultrasound spectroscopy was used to measure the elastic properties of pure polycrystalline (239)Pu in the gamma-phase. Shear and longitudinal elastic moduli were measured simultaneously and the bulk modulus was computed from them. A smooth, linear, and large decrease in all elastic moduli with increasing temperature was observed. The Poisson ratio was calculated and an increase from 0.242 at 519 K to 0.252 at 571 K was found. These measurements on extremely well-characterized pure Pu are in agreement with other reported results where overlap occurs. We calculated an approximate Debye temperature Theta(D)=144 K. Determined from the temperature variation in the bulk modulus, gamma-Pu shows the same Gruneisen parameter as copper.

Quantum phase transition in the magnetic-field-induced normal state of optimum-doped high-Tc cuprate superconductors at low temperatures

A 60 T magnetic field suppresses the superconducting transition temperature T_{c} in La_{2-p}Sr_{p}CuO_{4} to reveal a Hall number anomaly, which develops only at temperatures below zero-field T_{c} and peaks at the exact location of p that maximizes T_{c}. The anomaly bears a striking resemblance to observations in Bi_{2}Sr_{2-x}La_{x}CuO_{6+delta}, suggesting a normal-state phenomenology common to the cuprates that underlies the high-temperature superconducting phase. The peak is ascribed to a Fermi surface reconstruction at a quantum phase transition near optimum doping that is coincident with the collapse of the pseudogap state.

Alpha-plutonium's polycrystalline elastic moduli over its full temperature range

alpha-plutonium's volume-corrected polycrystal elastic moduli were measured between 18 K and the upper limit of its occurrence, near 400 K. The two independent moduli for a polycrystal-bulk and shear-behave smoothly, indicating no phase transition. Both moduli show the same 50% increase on cooling, an order of magnitude larger than in other metals. The Debye temperature obtained from low-temperature elastic moduli, 207 K, significantly exceeds most previous estimates. The Gruneisen parameter gamma=5.3, obtained from the temperature dependence of the bulk modulus, is intermediate among previous estimates using other approaches, alpha-plutonium's Poisson ratio nu is low: 0.18, nearly temperature independent, and its small decrease on warming opposes usual behavior. The high gamma, large but equal bulk modulus and shear modulus fractional stiffening on cooling, and near-temperature-invariant nu are attributed to a single mechanism: 5-f electron localization-delocalization.

Bulk vs nanoscale WS2: finite size effects and solid-state lubrication

To investigate phonon confinement in nanoscale metal dichalcogenides, we measured the low-temperature specific heat of layered and nanoparticle WS2. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk counterpart. Further, it deviates from the usual T 3 dependence below 4 K due to finite size effects that eliminate long wavelength acoustic phonons and interparticle-motion entropy. This separation of nanoscale effects from T 3 dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape. We invoke relationships between the low-temperature T 3 phonon term, Young's modulus, and friction coefficient to assess the difference in the tribological properties. On the basis of this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic.

Current jets, disorder, and linear magnetoresistance in the silver chalcogenides

The inhomogeneous distribution of excess or deficient silver atoms lies behind the large and linear transverse magnetoresistance displayed by Ag2+/-delta Se and Ag2+/-delta, introducing spatial conductivity fluctuations with length scales independent of the cyclotron radius. We report a negative, nonsaturating longitudinal magnetoresistance up to at least 60 T, which becomes most negative where the bands cross and the effect of conductivity fluctuations is most acute. Thinning samples down to 10 microm suppresses the negative response, revealing the essential length scale in the problem and paving the way for designer magnetoresistive devices.

Monocrystal elastic constants of the negative-thermal-expansion compound zirconium tungstate (ZrW2O8)

We measured zirconium tungstate's elastic constants C(ij). This compound shows relatively soft, nearly isotropic elastic constants with normal Poisson ratios and no approach to Born instability. ZrW2O8 shows normal ambient-temperature elastic constants C(ij), but remarkable dC(ij)/dT that show dominant low-frequency acoustic-vibration modes. From the bulk modulus, we estimated the total ambient-temperature thermodynamic Grüneisen parameter as gamma = -1.2. The dB/dT slope gives a Grüneisen parameter gamma = -7. The 300-0 K bulk-modulus increase (40%) seems unprecedented and breaks Birch's law of corresponding states.

Quantum phase transitions in the cuprate superconductor Bi2Sr2-xLaxCuO6+delta

To elucidate a quantum phase transition (QPT) in Bi(2)Sr(2-x)La(x)CuO(6+delta), we measure charge and heat transport properties at very low temperatures and examine the following characteristics for a wide range of doping: normal-state resistivity anisotropy under 58 T, temperature dependence of the in-plane thermal conductivity kappa(ab), and the magnetic-field dependence of kappa(ab). It turns out that all of them show signatures of a QPT at the 1/8 hole doping. Together with the recent normal-state Hall measurements under 58 T that signified the existence of a QPT at optimum doping, the present results indicate that there are two QPTs in the superconducting doping regime of this material.

Experimental electronic heat capacities of alpha- and delta-plutonium: heavy-fermion physics in an element

We have measured the heat capacities of delta-Pu0.95Al0.05 and alpha-Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduc-tion of the electronic contribution to the heat capacity of delta-Pu0.95Al0.05; we find gamma=64+/-3 mJ K(-2) mol(-1) as T-->0. This is larger than that of any element and large enough for delta-Pu0.95Al0.05 to be classed as a heavy-fermion system. By contrast, gamma=17+/-1 mJ K(-2) mol(-1) in alpha-Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of delta-Pu0.95Al0.05, one or both of which may be associated with the formation of the alpha(')-martensitic phase. We suggest that the large gamma value of delta-Pu0.95Al0.05 may be caused by proximity to a quantum-critical point.

Megagauss sensors

Magnetic fields change the way that electrons move through solids. The nature of these changes reveals information about the electronic structure of a material and, in auspicious circumstances, can be harnessed for applications. The silver chalcogenides, Ag2Se and Ag2Te, are non-magnetic materials, but their electrical resistance can be made very sensitive to magnetic field by adding small amounts--just 1 part in 10,000--of excess silver. Here we show that the resistance of Ag2Se displays a large, nearly linear increase with applied magnetic field without saturation to the highest fields available, 600,000 gauss, more than a million times the Earth's magnetic field. These characteristics of large (thousands of per cent) and near-linear response over a large field range make the silver chalcogenides attractive as magnetic-field sensors, especially in physically tiny megagauss (10(6) G) pulsed magnets where large fields have been produced but accurate calibration has proved elusive. High-field studies at low temperatures reveal both oscillations in the magnetoresistance and a universal scaling form that point to a quantum origin for this material's unprecedented behaviour.

Metal-to-insulator crossover in the low-temperature normal state of Bi(2)Sr(2-x)La(x)CuO(6+delta)

We measure the normal-state in-plane resistivity of Bi(2)Sr(2-x)La(x)CuO(6+delta) single crystals at low temperatures by suppressing superconductivity with 60 T pulsed magnetic fields. With decreasing hole doping, we observe a crossover from a metallic to an insulating behavior in the low-temperature normal state. This crossover is estimated to occur near 1/8 doping, well inside the underdoped regime, and not at optimum doping as reported for other cuprates. The insulating regime is marked by a logarithmic temperature dependence of the resistivity over two decades of temperature, suggesting that a peculiar charge localization is common to the cuprates.