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Two-dimensional van der Waals magnets with multiple functionalities are becoming increasingly important for emerging technologies in spintronics and valleytronics. Application of external pressure is one method to cleanly explore the underlying physical mechanisms of the intrinsic magnetism. In this paper, the magnetic, electronic, and structural properties of van der Waals-layered, Fe-deficient ${\mathrm{Fe}}_{3\ensuremath{-}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ are investigated. Magnetotransport measurements show a monotonic decrease in the Curie temperature $({T}_{C})$ and the magnetic moment with increasing pressure up to 13.9 GPa. The electrical resistance of ${\mathrm{Fe}}_{3\ensuremath{-}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ shows a change from metallic to a seemingly nonmetallic behavior with increasing pressure. High-pressure angle dispersive powder x-ray diffraction shows a monotonic compression of the unit cell and a reduction of the volume by $\ensuremath{\sim}25%$ with no evidence of structural phase changes up to 29.4(4) GPa. We suggest that the decrease in the ${T}_{C}$ due to pressure results from increased intralayer coupling and delocalization that leads to a change in the exchange interaction.

Producing gram quantities of uranium metal in a controlled manner by traditional methods is challenging due to the complex chemistry of precursor material and extreme thermal requirements. In this article, a novel approach is reported that combines modeling and an advanced experimental technique for extracting uranium from a uranium-containing compound. Using uranium nitride as an example, a computational thermodynamic approach identified a decomposition pathway to convert uranium nitride to uranium metal at temperatures exceeding 2500 K under conditions of rapid material cooling. To realize these extreme conditions, laser-induced heating, which enables fine control of process location and rapid cooling, was utilized for high-temperature modification of material. Uranium nitride was irradiated by a controlled laser under several gaseous conditions including high-vacuum, argon, and nitrogen environments, resulting in uranium metal at yields up to 96%. The complete decomposition leading to pure uranium metal occurs at the high temperature surface region, where laser-based heating induces a surface depression and molten pool of material. The observed kinetic phase behaviors in this study fundamentally differ from previous uranium decomposition studies where small uranium metal precipitates from the nitride bulk are formed at the surface of uranium nitride.

We report the observation of the pressure-induced, fully collapsed tetragonal phase of $\mathrm{CaRbF}{\mathrm{e}}_{4}\mathrm{A}{\mathrm{s}}_{4}$ for $P\ensuremath{\sim}22\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ via high-pressure x-ray diffraction and magnetotransport measurements. The x-ray measurements, along with resistivity measurements, show that there is an initial half-collapsed tetragonal phase for $6lPl22\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, in which superconductivity is continuously suppressed from ${T}_{\mathrm{c}}=35\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at $P=3.1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ to ${T}_{\mathrm{c}}l2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $P\ensuremath{\ge}17.2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, as well as signs of the fully collapsed tetragonal phase near $P=22\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Density functional calculations suggest that both of these transitions are driven by increased As-As bonding, first across the Ca layer, and then at the second transition, across the Rb layer. Although electrical resistivity measurements in the fully collapsed tetragonal phase do not show superconductivity, there is a change in the slope of both the Hall coefficient and the longitudinal resistance near 22 GPa, suggesting a strong correlation between the electronic and lattice degrees of freedom in this new iron-based superconductor.

Author(s): Brubaker, ZE; Stillwell, RL; Chow, P; Xiao, Y; Kenney-Benson, C; Ferry, R; Popov, D; Donald, SB; Soderlind, P; Campbell, DJ; Paglione, J; Huang, K; Baumbach, RE; Zieve, RJ; Jeffries, JR | Abstract: We report a comprehensive structural and valence study of the intermediate valent materials YbNiGa4 and YbNiIn4 under pressures up to 60 GPa. YbNiGa4 undergoes a smooth volume contraction and shows steady increase in Yb valence with pressure, though the Yb valence reaches saturation around 25 GPa. In YbNiIn4, a change in pressure dependence of the volume and a peak in Yb valence suggest that a pressure-induced electronic topological transition occurs around 10-14 GPa. In the pressure region where YbNiIn4 and YbNiGa4 possess similar Yb-Yb spacings, the Yb valence reveals a precipitous drop. This drop is not captured by density functional theory calculations and implies that both the lattice degrees of freedom and the chemical environment play an important role in establishing the valence of Yb.

The early-time kinetic behavior of species in a plume produced from laser ablation of U3Si2 in an environment containing 2% O2 is characterized using time-resolved absorption spectroscopy. The UO band around 593.55 nm and the SiO band around 230 nm is observed as well as various atomic and ionic uranium transitions. Temperatures and concentrations of these species are tracked and reported as well.

We report the pressure dependence of the N\'eel temperature ${T}_{N}$ up to $P\ensuremath{\approx}27$ GPa for the recently discovered itinerant antiferromagnet (IAFM) TiAu. The ${T}_{N}(P)$ phase boundary exhibits unconventional behavior in which the N\'eel temperature is enhanced from ${T}_{N}\ensuremath{\approx}33$ K at ambient pressure to a maximum of ${T}_{N}\ensuremath{\approx}35$ K occurring at $P\ensuremath{\approx}5.5$ GPa. Upon a further increase in pressure, ${T}_{N}$ is monotonically suppressed to $\ensuremath{\sim}22$ K at $P\ensuremath{\approx}27$ GPa. We also find a crossover in the temperature dependence of the electrical resistivity $\ensuremath{\rho}$ in the antiferromagnetic (AFM) phase that is coincident with the peak in ${T}_{N}(P)$, such that the temperature dependence of $\ensuremath{\rho}={\ensuremath{\rho}}_{0}+{A}_{n}{T}^{n}$ changes from $n\ensuremath{\approx}3$ during the enhancement of ${T}_{N}$ to $n\ensuremath{\approx}2$ during the suppression of ${T}_{N}$. Based on an extrapolation of the ${T}_{N}(P)$ data to a possible pressure-induced quantum critical point, we estimate the critical pressure to be ${P}_{c}\ensuremath{\approx}45$ GPa.

In pulsed magnetic fields up to 65 T and at temperatures below the N\'eel transition, our magnetization and magnetostriction measurements reveal a field-induced metamagneticlike transition that is suggestive of an antiferromagnetic to ferrimagnetic ordering. Our data also suggest a change in the nature of this metamagneticlike transition from second- to first-order-like near a tricritical point at ${T}_{\mathrm{tc}}\ensuremath{\sim}145\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${H}_{\mathrm{c}}\ensuremath{\sim}52\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. At high fields for $Hg{H}_{\mathrm{c}}$ we found a decreased magnetic moment roughly half of the moment determined by neutron powder diffraction. We propose that the decreased moment and lack of saturation at high fields indicate the presence of a field-induced ferrimagnetic state above the tricritical point of the H-T phase diagram for $\mathrm{US}{\mathrm{b}}_{2}$.

Extreme magnetoresistance (XMR) in topological semimetals is a recent discovery which attracts attention due to its robust appearance in a growing number of materials. To search for a relation between XMR and superconductivity, we study the effect of pressure on LaBi. By increasing pressure, we observe the disappearance of XMR followed by the appearance of superconductivity at $P\ensuremath{\approx}3.5$ GPa. We find a region of coexistence between superconductivity and XMR in LaBi in contrast to other superconducting XMR materials. The suppression of XMR is correlated with increasing zero-field resistance instead of decreasing in-field resistance. At higher pressures, $P\ensuremath{\approx}11$ GPa, we find a structural transition from the face-centered cubic lattice to a primitive tetragonal lattice, in agreement with theoretical predictions. The relationship between extreme magnetoresistance, superconductivity, and structural transition in LaBi is discussed.

We have studied the Ce valence as a function of pressure in CeRhIn5 at 300 K and at 22 K using x-ray absorption spectroscopy in partial fluorescent yield mode. At room temperature, we found no detectable change in Ce valence greater than 0.01 up to a pressure of 5.5 GPa. At 22 K, the valence remains robust against pressure below 6 GPa, in contrast to the predicted valence crossover at P=2.35 GPa. This work yields an upper limit for the change in Ce-valence and suggests that the critical valence fluctuation scenario, in its current form, is unlikely., Comment: 6 pages, 3 figures

The new oxytelluride Ba3Yb2O5Te was obtained from an alkaline earth flux. Ba3Yb2O5Te crystallizes in the tetragonal space group P4/mmm (#123), with a=4.3615(3) A and c=11.7596(11) A, Z=1. The structure combines two distinct building blocks, a Ba2Yb2O5 perovskite-like double layer with square bipyramidal coordination of the ytterbium ions, and a CsCl-type BaTe layer. Short range magnetic order is apparent at below 5 K, with the magnetic behavior above this temperature dominated by crystal field effects. The structure may be considered as an analog to the Ruddlesden–Popper phases, where the NaCl-type layer has been replaced by the CsCl-type layer. The two-dimensional magnetic behavior is expected based on the highly anisotropic nature of the structure.

We use neutron scattering to compare the magnetic excitations in the hidden order (HO) and antiferromagnetic (AFM) phases in ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{Si}}_{2}$ as a function of Fe concentration. The magnetic excitation spectra change significantly between $x=0.05$ and $x=0.10$, following the enhancement of the AFM ordered moment, in good analogy to the behavior of the parent compound under applied pressure. Prominent lattice-commensurate low-energy excitations characteristic of the HO phase vanish in the AFM phase. The magnetic scattering is dominated by strong excitations along the Brillouin zone edges, underscoring the important role of electron hybridization to both HO and AFM phases and the similarity of the underlying electronic structure. The stability of the AFM phase is correlated with enhanced local-itinerant electron hybridization.

The material USb2 is a correlated, moderately heavy-electron compound within the uranium dipnictide (UX2) series. It is antiferromagnetic with a relatively high transition temperature TN = 204K and a large U-U separation. While the uranium atoms in the lighter dipnictides are considered to be localized, those of USb2 exhibit hybridization and itineracy, promoting uncertainty as to the continuity of the magnetic order within the UX2. We have explored the evolution of the magnetic order by employing magnetotransport measurements as a function of pressure and temperature. We find that the TN in USb2 is enhanced, moving towards that of its smaller sibling UAs2. But, long before reaching a TN as high as UAs2, the antiferromagnetism of USb2 is abruptly destroyed in favor of another magnetic ground state. We identify this pressure-induced ground state as being ferromagnetic based on the appearance of a strong anomalous Hall effect in the transverse resistance in magnetic field. At last with pressure, this emergent ferromagnetic state is suppressed and ultimately destroyed in favor of a non-Fermi-liquid ground state.

Using high-pressure magnetotransport techniques we have discovered superconductivity in Bi2Te, a member of the infinitely adaptive (Bi2)m(Bi2Te3)n series, whose end members, Bi and Bi2Te3, can be tuned to display topological surface states or superconductivity. Bi2Te has a maximum Tc = 8.6 K at P = 14.5 GPa and goes through multiple high pressure phase transitions, ultimately collapsing into a bcc structure that suggests a universal behavior across the series. High-pressure magnetoresistance and Hall measurements suggest a semi-metal to metal transition near 5.4 GPa, which accompanies the hexagonal to intermediate phase transition seen via x-ray diffraction measurements. In addition, the linearity of Hc2 (T) exceeds the Werthamer-Helfand-Hohenberg limit, even in the extreme spin-orbit scattering limit, yet is consistent with other strong spin-orbit materials. Furthermore, considering these results in combination with similar reports on strong spin-orbit scattering materials seen in the literature, we suggest the need for a new theory that can address the unconventional nature of their superconducting states.

R(33)Fe(14-x)Al(x+y)B(25-y)C(34) (R = La or Ce; x ≤ 0.9; y ≤ 0.2) and R(33)Fe(13-x)Al(x)B(18)C(34) (R = Ce or Pr; x0.1) were synthesized from reactions of iron with boron, carbon, and aluminum in R-T eutectic fluxes (T = Fe, Co, or Ni). These phases crystallize in the cubic space group Im3m (a = 14.617(1) Å, Z = 2, R(1) = 0.0155 for Ce(33)Fe(13.1)Al(1.1)B(24.8)C(34), and a = 14.246(8) Å, Z = 2, R(1) = 0.0142 for Ce(33)Fe(13)B(18)C(34)). Their structures can be described as body-centered cubic arrays of large Fe(13) or Fe(14) clusters which are capped by borocarbide chains and surrounded by rare earth cations. The magnetic behavior of the cerium-containing analogs is complicated by the possibility of two valence states for cerium and possible presence of magnetic moments on the iron sites. Temperature-dependent magnetic susceptibility measurements and Mössbauer data show that the boron-centered Fe(14) clusters in Ce(33)Fe(14-x)Al(x+y)B(25-y)C(34) are not magnetic. X-ray photoelectron spectroscopy data indicate that the cerium is trivalent at room temperature, but the temperature dependence of the resistivity and the magnetic susceptibility data suggest Ce(3+/4+) valence fluctuation beginning at 120 K. Bond length analysis and XPS studies of Ce(33)Fe(13)B(18)C(34) indicate the cerium in this phase is tetravalent, and the observed magnetic ordering at T(C) = 180 K is due to magnetic moments on the Fe(13) clusters.

The authors report the synthesis of nanocrystalline, alloyed PbEuS, a potentially important dilute magnetic semiconductor. The thermolysis of mixed precursors has been adapted for the formation of homogeneous alloyed nanocrystals. X-ray diffraction and magnetization measurements of ternary PbEuS nanocrystals provide convincing evidence that no phase separation occurs in these nanomaterials for europium concentrations up to x = 0.17. X-ray photoelectron spectroscopy provides the atomic composition for PbEuS alloy nanocrystals juxtaposed with the ratio of the starting precursors.

Single crystals of CaMgSi were produced using the metal flux synthesis method in a Mg/Al 1:1 mixture. The large rod-shaped crystals measure up to 7 mm in length. This phase crystallizes with the orthorhombic TiNiSi structure type (space group Pnma; a = 7.4752(2) A, b = 4.42720(10) A, c = 8.3149(2) A; R1 = 0.021). Despite its relationship to semiconducting Zintl phases Mg2Si and Ca2Si, CaMgSi is metallic at room temperature; this produces a positive (∼160 ppm) 29Si MAS NMR chemical shift and is supported by DOS calculations. A metal to semimetal electronic transition at around 50 K is evident in the resistivity, magnetic susceptibility, and electron paramagnetic resonance measurements. Low temperature powder X-ray diffraction data indicates that a structural distortion accompanies this transition. The electronic heat capacity coefficient (0.4695 mJ/mol·K2) determined from low temperature heat capacity data supports the designation of CaMgSi as a semimetal at low temperature. The hydrogen storage capacity o...

We have performed high-pressure magnetotransport and x-ray diffraction measurements on ferromagnetic $\mathrm{LaC}{\mathrm{o}}_{5}$, confirming the theoretically predicted electronic topological transition driving the magnetoelastic collapse seen in the related compound $\mathrm{YC}{\mathrm{o}}_{5}$. Our x-ray diffraction results show an anisotropic lattice collapse of the $c$ axis near 10 GPa that is also commensurate with a change in the majority charge carriers evident from high-pressure Hall effect measurements. The coupling of the electronic, magnetic, and lattice degrees of freedom is further substantiated by the evolution of the anomalous Hall effect, which couples to the magnetization of the ordered state of $\mathrm{LaC}{\mathrm{o}}_{5}$.

We have observed a massive reconstruction of the Fermi surface of single crystal chromium as a function of high pressure and high magnetic fields caused by the spin-flip transition, with multiple new orbits appearing above 0.93 GPa. In addition, some orbits have field-induced effective masses of \ensuremath{\sim}0.06--0.07 ${m}_{e}$, seen only at high magnetic fields. Based on the temperature insensitivity displayed by the oscillation amplitudes at these frequencies, we attribute the orbits to quantum interference rather than to Landau quantization.

Measuring surface conductivity we have observed the evolution of Shubnikov de Haas oscillations under quasihydrostatic pressure for the pnictide parent compound BaFe${}_{2}$As${}_{2}$. Prior results in the reconstructed state have observed small pockets which emerge from zone folding as a result of structural changes with cooling. For pressures below 20 kbar, both Fermi surface orbits grow in size. The effective masses increase with pressure suggesting enhanced correlation in the system, and a series of magnetic breakdown orbits are observed confirming that band structure calculations setting them in close proximity are correct.

Bismuth-Antimony alloys have been shown to have high ZT values below room temperature, especially for single crystals. For polycrystalline samples, impurity doping and magnetic field have proven to be powerful tools in the search for understanding and improving thermoelectric performance. Nanopolycrystalline Bi0.88Sb0.12 doped with 0.05, 0.5 and 3 % Ce were prepared by ball milling and dc hot pressing techniques. Electrical resistivity, Seebeck coefficient, thermal conductivity, carrier concentration, mobility, and magnetization are measured in a temperature range of 5-350 K and in magnetic fields up to 9 Tesla. The effects of Ce doping on the thermoelectric properties of Bi0.88Sb0.12 in zero magnetic field are discussed.

A plastic turnbuckle diamond anvil cell (DAC) and nonmetallic gasket have been developed for pulsed magnetic field studies to address issues of eddy current heating and Lorentz forces in metal cells. The plastic cell evolved from our 6.3 mm metal diameter turnbuckle DAC that was designed in 1993 to rotate in the 9 mm sample space of Quantum Design's MPMS. Attempts to use this metal DAC in pulsed magnetic fields caused the sample temperature to rise to T>70 K, necessitating the construction of a nonconductive cell and gasket. Pressures of 3 GPa have been produced in the plastic cell with 0.8 mm culets in an optical study conducted at T = 4 K. Variations of the cell are now being used for fermiology studies of metallic systems in pulsed magnetic fields that have required the development of a rotator and a special He-3 cryostat which are also discussed.

The flat bands in bilayer graphene(BLG) are sensitive to electric fields E\bot directed between the layers, and magnify the electron-electron interaction effects, thus making BLG an attractive platform for new two-dimensional (2D) electron physics[1-5]. Theories[6-16] have suggested the possibility of a variety of interesting broken symmetry states, some characterized by spontaneous mass gaps, when the electron-density is at the carrier neutrality point (CNP). The theoretically proposed gaps[6,7,10] in bilayer graphene are analogous[17,18] to the masses generated by broken symmetries in particle physics and give rise to large momentum-space Berry curvatures[8,19] accompanied by spontaneous quantum Hall effects[7-9]. Though recent experiments[20-23] have provided convincing evidence of strong electronic correlations near the CNP in BLG, the presence of gaps is difficult to establish because of the lack of direct spectroscopic measurements. Here we present transport measurements in ultra-clean double-gated BLG, using source-drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the CNP. The gap can be closed by an electric field E\bot \sim13 mV/nm but increases monotonically with a magnetic field B, with an apparent particle-hole asymmetry above the gap, thus providing the first mapping of the ground states in BLG., 4 figures

We have performed de Haas-van Alphen measurements of the Fermi surface of alpha-uranium single crystals at ambient pressure within the alpha-3 charge density wave (CDW) state from 0.020 K - 10 K and magnetic fields to 35 T using torque magnetometry. The angular dependence of the resulting frequencies is described. Effective masses were measured and the Dingle temperature was determined to be 0.74 K +/- 0.04 K. The observation of quantum oscillations within the alpha-3 CDW state gives new insight into the effect of the charge density waves on the Fermi surface. In addition we observed no signature of superconductivity in either transport or magnetization down to 0.020 K indicating the possibility of a pressure-induced quantum critical point that separates the superconducting dome from the normal CDW phase., Comment: 11 pages, 4 figures, 3 tables