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In recent times, transformer networks have achieved state-of-the-art performance in a wide range of natural language processing tasks. Here we present a workflow based on the fine-tuning of BERT models for different downstream tasks, which results in the automated extraction of structured information from unstructured natural language in scientific literature. Contrary to existing methods for the automated extraction of structured compound-property relations from similar sources, our workflow does not rely on the definition of intricate grammar rules. Hence, it can be adapted to a new task without requiring extensive implementation efforts and knowledge. We test our data-extraction workflow by automatically generating a database for Curie temperatures and one for band gaps. These are then compared with manually-curated datasets and with those obtained with a state-of-the-art rule-based method. Furthermore, in order to showcase the practical utility of the automatically extracted data in a material-design workflow, we employ them to construct machine-learning models to predict Curie temperatures and band gaps. In general we find that, although more noisy, automatically extracted datasets can grow fast in volume and that such volume partially compensates for the inaccuracy in downstream tasks., Comment: 19 pages, 11 figures

Technologies that function at room temperature often require magnets with a high Curie temperature, $T_\mathrm{C}$, and can be improved with better materials. Discovering magnetic materials with a substantial $T_\mathrm{C}$ is challenging because of the large number of candidates and the cost of fabricating and testing them. Using the two largest known data sets of experimental Curie temperatures, we develop machine-learning models to make rapid $T_\mathrm{C}$ predictions solely based on the chemical composition of a material. We train a random forest model and a $k$-NN one and predict on an initial dataset of over 2,500 materials and then validate the model on a new dataset containing over 3,000 entries. The accuracy is compared for multiple compounds' representations ("descriptors") and regression approaches. A random forest model provides the most accurate predictions and is not improved by dimensionality reduction or by using more complex descriptors based on atomic properties. A random forest model trained on a combination of both datasets shows that cobalt-rich and iron-rich materials have the highest Curie temperatures for all binary and ternary compounds. An analysis of the model reveals systematic error that causes the model to over-predict low-$T_\mathrm{C}$ materials and under-predict high-$T_\mathrm{C}$ materials. For exhaustive searches to find new high-$T_\mathrm{C}$ materials, analysis of the learning rate suggests either that much more data is needed or that more efficient descriptors are necessary., Comment: 9 pages, 11 figures, accepted to Applied Physics Letters, special issue "Accelerate Materials Discovery and Phenomena"

Chemical modifications such as intercalation can be used to modify surface properties or to further functionalize the surface states of topological insulators. Using ambient pressure X-ray photoelectron spectroscopy, we report copper migration in $\text{Cu}_{x}\text{Bi}_2\text{Se}_3$, which occurs on a timescale of hours to days after initial surface cleaving. The increase in near-surface copper proceeds along with the oxidation of the sample surface and large changes in the selenium content. These complex changes are further modelled with core-level spectroscopy simulations, which suggest a composition gradient near the surface which develops with oxygen exposure. Our results shed light on a new phenomenon that must be considered for intercalated topological insulators$\unicode{x2014}$and intercalated materials in general$\unicode{x2014}$that surface chemical composition can change when specimens are exposed to ambient conditions.

New determination of the magnetic anisotropy from single crystals of (Fe1-xCox)2B alloys are presented. The anomalous temperature dependence of the anisotropy constant is discussed using the standard Callen-Callen theory, which is shown to be insufficient to explain the experimental results. A more material specific study using first-principles calculations with disordered moments approach gives a much more consistent interpretation of the experimental data. Since the intrinsic properties of the alloys with x=0.3-0.35 are promising for permanent magnets applications, initial investigation of the extrinsic properties are described, in particular the crystallization of melt spun ribbons with Cu, Al, and Ti additions. Previous attempts at developing a significant hysteresis have been unsuccessful in this system. Our melt-spinning experiment indicates that this system shows rapid crystallization.

The Ce (Formula presented.) Mg (Formula presented.) Co (Formula presented.) system evolves from a Pauli paramagnetic ground state for x = 0 to a ferromagnetic ground state for (Formula presented.) in single-phase, polycrystalline samples [Lamichhane, V. Taufour, A. Palasyuk, Q. Lin, S.L. Budko, and P.C. Canfield, (Formula presented.) : transformation of a Pauli paramagnet into a strong permanent magnet, Phys. Rev. Appl. 9 (2018), p. 024023]. In order to better understand this behaviour, single-crystalline samples of Ce (Formula presented.) Mg (Formula presented.) Co (Formula presented.) for x = 0.01, 0.16, 0.24, 0.35, 0.43 and 0.50 were grown using the flux growth technique, and electrical transport and magnetic properties were studied. The (Formula presented.) -x phase diagram we infer shows that the system has a quantum phase transition near x = 0.35, transforming to a ferromagnetic ground state.

Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBa2Cu3O7-δ (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation.

Solid state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here we report a simple, scalable approach allowing for total control of the superconducting transition in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-��}$ (YBCO) films via a chemically-driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective ionic control of superconductivity in oxides, an interface induced effect that goes well into the quasi-bulk regime, opening up possibilities for electric field manipulation., 26 pages, 5 figures

Recent theoretical and experimental studies have shown that ferromagnetic quantum criticality is always avoided in clean systems. Two possibilities have been identified. In the first scenario, the ferromagnetic transition becomes of the first order at a tricritical point before being suppressed. A wing structure phase diagram is observed indicating the possibility of a new type of quantum critical point under magnetic field. In a second scenario, a transition to a modulated magnetic phase occurs. Our recent studies on the compound LaCrGe3 illustrate a third scenario where not only a new magnetic phase occurs, but also a change of order of the transition at a tricritical point leading to a wing-structure phase diagram. Careful experimental study of the phase diagram near the tricritical point also illustrates new rules near this type of point.

We report on the synthesis of single crystalline and polycrystalline samples of Ce$_{3-x}$Mg$_x$Co$_9$ solid solution ($0\leq x \lesssim 1.4$) and characterization of their structural and magnetic properties. The crystal structure remains rhombohedral in the whole composition range and Mg partially replaces Ce in the 6\textit{c} site of the CeCo$_3$ structure. Ferromagnetism is induced by Mg substitutions starting as low as $x=0.18$ and reaching a Curie temperature as high as $450$\,K for $x=1.35$. Measurements on single crystals with $x=1.34$ and $T_\textrm{C}=440$\,K indicate an axial magnetic anisotropy with the anisotropy field of $6$\,T and a magnetization of $6$\,$\mu_B/$f.u. at $300$\,K. Coercicity is observed in the polycrystalline samples consistent with the observed axial magnetic anisotropy. Our discovery of ferromagnetism with large axial magnetic anisotropy induced by substituting a rare-earth element by Mg is a very promising result in the search of inexpensive permanent-magnet materials and suggests other non-magnetic phases, similar to CeCo$_3$, may also conceal nearby ferromagnetic phases., Comment: Physical Review Applied 2018

In this paper, we present a coupled experimental/theoretical investigation of pressure effect on the ferromagnetism of LaCrGe3 and LaCrSb3 compounds. The magnetic, electronic, elastic and mechanical properties of LaCrGe3 and LaCrSb3 at ambient condition are studied by first-principles density functional theory calculations. The pressure dependences of the magnetic properties of LaCrGe3 and LaCrSb3 are also investigated. The ferromagnetism in LaCrGe3 is rather fragile with a ferro- to paramagnetic transition at a relatively small pressure (around 7 GPa from our calculations, and 2 GPa in experiments). The key parameter controlling the magnetic properties of LaCrGe3 is found to be the proximity of the Cr DOS to the Fermi surface, a proximity that is strongly correlated to the distance between Cr atoms along the c-axis, suggesting that there would be a simple way to suppress magnetism in systems with one dimensional arrangement of magnetic atoms. By contrast, the ferromagnetism in LaCrSb3 is not fragile. Our calculation results are consistent with our experimental results and demonstrate the feasibility of using first-principles calculations to aid experimental explorations in screening for materials with fragile magnetism.

The synthesis and crystallographic, thermodynamic and transport properties of single crystalline Rh$_{9}$In$_4$S$_4$ were studied. The resistivity, magnetization and specific heat measurements all clearly indicate bulk superconductivity with a critical temperature, $T_{c}\sim$ 2.25 K. The Sommerfeld coefficient $\gamma$ and the Debye temperature($\varTheta_{\textrm{D}}$) were found to be 34 mJ mol$^{-1}$ K$^{-2}$ and 217 K respectively. The observed specific heat jump, $\Delta C/\gamma T_{c}$ = 1.66, is larger than the expected BCS weak coupling value of 1.43. Ginzburg-Landau (GL) ratio of the low temperature GL-penetration depth, $\lambda_\text{GL}$$\approx$ 5750\AA, to the GL-coherence length, $\xi_\text{GL}$$\approx$ 94\AA, is large: $\kappa$ $\sim$ 60. Furthermore, we observed a peak effect in the resistivity measurement as a function of both temperature and magnetic field.

We study the evolution of the Kondo effect in heavy fermion compounds, Yb(Fe$_{1-x}$Co$_{x}$)$_{2}$Zn$_{20}$ (0$\leqslant$ x $\leqslant$ 1), by means of temperature-dependent electric resistivity and specific heat. The ground state of YbFe$_2$Zn$_{20}$ can be well described by a Kondo model with degeneracy $N$ = 8 and a $T_K\sim$30 K. In the presence of a very similar total CEF splitting with YbFe$_2$Zn$_{20}$, the ground state of YbCo$_2$Zn$_{20}$ is close to a Kondo state with degeneracy $N$ = 2 and a much lower $T_K\sim$ 2 K. Upon Co substitution, the coherence temperature of YbFe$_2$Zn$_{20}$ is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For 0.4$\lesssim$ x $\lesssim$ 0.9, the ground state remains roughly the same which can be qualitatively understood by Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence observable in resistivity within this substitution range down to 500 mK. The coherence re-appears at around x$\gtrsim$ 0.9 and the coherence temperature increases with higher Co concentration levels., Comment: 8 pages, 6 figures

The temperature-pressure phase diagram of the ferromagnet LaCrGe$_3$ is determined for the first time from a combination of magnetization, muon-spin-rotation and electrical resistivity measurements. The ferromagnetic phase is suppressed near $2.1$~GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFM$_Q$. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors $Q$ allowing for the potential of an ordering wave vector evolving from $Q=0$ to finite $Q$, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe$_3$ is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.

We study magnetic domains in Nd2Fe14B single crystals using high resolution magnetic force mi- croscopy (MFM). Previous MFM studies and small angle neutron scattering experiments suggested the presence of nano-scale domains in addition to optically detected micrometer-scale ones. We find, in addition to the elongated, wavy nano-domains reported by a previous MFM study, that the micrometer size, star shape fractal pattern is constructed of an elongated network of nano-domains ~20 nm in width, with resolution-limited domain walls thinner than 2 nm. While the microscopic domains exhibit significant resilience to an external magnetic field, some of the nano-domains are sensitive to the magnetic field of the MFM tip., 4 pages, 5 figures

We use tunable laser based Angle Resolved Photoemission Spectroscopy to study the electronic structure of the multi-band superconductor, MgB2. These results form the base line for detailed studies of superconductivity in multi-band systems. We find that the magnitude of the superconducting gap on both sigma bands follows a BCS-like variation with temperature with Delta0 ~7 meV. The value of the gap is isotropic within experimental uncertainty and in agreement with pure a s-wave pairing symmetry. We also observe in-gap states confined to kF of the sigma band that occur at some locations of the sample surface. The energy of this excitation, ~3 meV, is inconsistent with scattering from the pi band., 6 pages, 4 figures

The well-known BaAl4-type structure consists of three ordered ternary derivatives, i.e., BaNiSn3-(I4mm), ThCr2Si2-(I4/mmm), and CaBe2Ge2-type (P4/nmm). Few systems, such as Ba-Au-Sn, have been confirmed to manifest all three types as a function of valence electron count. In this work, the SrAuxSn4-xsolid solution at the 600 ° C section was studied thoroughly using both single crystal and powder X-ray diffraction. The crystal structures and phase width for the CaBe2Ge2-type SrAuxSn4-xsolid solution were established to be α = 4.6528(2)-4.6233(3) Å and c = 11.3753(4)-11.2945(10) Å for x ≈ 1.65(1)-2.19(1). In the structure of SrAu2Sn2, no Au/Sn mixing was found, but for x < 2 compositions, Au/Sn mixings were only located at the Wyckoff 2α (3/4 1/4 0) site and for x > 2 compositions, at the 2b (3/4 1/4 1/2) site. Differential scanning calorimetry (DSC) analyses indicated that no phase transition occurred for the CaBe2Ge2-type SrAuxSn4-xphase up to 950 ° C. Attempts to synthesize the ThCr2Si2-and BaNiSn3-type SrAuxSn4-xphases under the same reaction conditions were unsuccessful, and the BaNiSn3-type phase could not be attained even at a pressure of 3 GPa. The instability of a BaNiSn3-type "SrAuSn3" was investigated by both DSC measurements and first principles electronic structure calculations.

Single crystals of Bi$_{2} $Rh$ _{3}$S$ _{2}$ and Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ were synthesized by solution growth and the crystal structures, thermodynamic and transport properties of both compounds were studied. In the case of Bi$_{2} $Rh$ _{3}$S$ _{2}$, a structural first-order transition at around 165 K is identified by single crystal diffraction experiments, with clear signatures visible in resistivity, magnetization and specific heat data. No superconducting transition for Bi$_{2} $Rh$ _{3}$S$ _{2}$ was observed down to 0.5 K. In contrast, no structural phase transition at high temperature was observed for Bi$_{2} $Rh$ _{3.5}$S$ _{2}$, however bulk superconductivity with a critical temperature, $T_{c}\approx$ 1.7 K was observed. The Sommerfeld coefficient $\gamma$\ and the Debye temperature($\varTheta_{\textrm{D}}$) were found to be 9.4 mJ mol$^{-1}$ K$^{-2}$ and 209 K respectively for Bi$_{2} $Rh$ _{3}$S$ _{2}$, and 22 mJ mol$^{-1}$ K$^{-2}$ and 196 K respectively for Bi$_{2} $Rh$ _{3.5}$S$ _{2}$. Study of the specific heat in the superconducting state of Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ suggests that Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ is a weakly coupled, BCS superconductor., Comment: 8 pages, 11 figures

We report measurements of electrical resistivity under pressure to 5.8 GPa, magnetization to 6.7 GPa,and ac susceptibility to 7.1 GPa in KFe2As2. The previously reported change of slope in the pressure dependence of the superconducting transition temperature Tc(p) at a pressure p0=1.8 GPa is confirmed, and Tc(p) is found to be nearly constant above p0 up to 7.1 GPa. The T-p phase diagram is very sensitive to the pressure conditions as a consequence of the anisotropic uniaxial pressure dependence of Tc. Across p0, a change in the behavior of the upper critical field is revealed through a scaling analysis of the slope of Hc2 with the effective mass as determined from the A coefficient of the T2 term of the temperature-dependent resistivity. We show that this scaling provides a quantitative test for the changes of the superconducting gap structure and suggests the development of a kz modulation of the superconducting gap above p0 as a most likely explanation.

We report synthesis of hexagonal LaVxCr1-xGe3 (x = 0-0.21, 1.00) single crystals, and present a systematic study of this series by measurements of temperature and field dependent magnetic susceptibility, magnetization, resistivity, and specific heat. Ferromagnetism has been observed for x = 0-0.21, and the system manifests a strong axial anisotropy in its ordered state. The decrease of the saturated moment and the effective moment per Cr with the increase of V-concentration suggests this is an itinerant ferromagnetic system. The Curie temperature declines monotonically as the V-concentration increases. Single crystalline samples could only be grown for x-values up to 0.21 for which the transition temperature was suppressed down to 36 K. Although we could not fully suppress TC via V-substitution, for x = 0.16, we performed magnetization measurements under pressure. The ferromagnetic state is suppressed under pressure at an initial rate of dTC/dp = -11.7 K/GPa and vanishes by 3.3 GPa.

The aim of this study is to compare the magnetic behavior of URu2Si2 under uniaxial stress along the a-axis with the behavior under hydrostatic pressure. Both are very similar, but uniaxial stress presents a critical stress {\sigma}xa smaller (0.33(5)GPa) than the hydrostatic critical pressure px =0.5 GPa where the ground state switches from HO (hidden order) to AF (antiferromagnetic) ground state. From these critical values and from Larmor neutron diffraction (LND), we conclude that the magnetic properties are governed by the shortest U-U distance in the plane (a lattice parameter). Under stress, the orthorhombic unit cell stays centered. A key point shown by this study is the presence of a threshold for the uniaxial stress along the a-axis before the appearance of the large AF moment which indicates no-mixture of order parameter (OP) between the HO ground state and the AF one as under hydrostatic pressure. The two most intense longitudinal magnetic excitations at Q0=(1,0,0) and Q1=(0.6,0,0) were measured in the HO state: the excitation at Q0 decreases in energy while the excitation at Q1 increases in energy with the uniaxial stress along the a-axis. The decrease of the energy of the excitation at Q0 seems to indicate a critical energy gap value of 1.2(1) meV at {\sigma}xa. A similar value was derived from studies under hydrostatic pressure at px., Comment: 22 pages, 10 figures, submitted to PRB

Cette thèse porte sur l'étude du composé supraconducteur ferromagnétique UGe2. La croissance de monocristaux de UGe2 a été réalisée dans un four tétra-arc par la technique du tirage Czochralski. Cette technique a également servie à l'obtention d'autres composés à base d'uranium, notament UCoGe et URu2Si2. Pour la première fois, la structure avec des ailes (wings) du diagramme de phase de UGe2 a été vérifiée expérimentalement. Cette observation est une conséquence d'une température de transition ferromagnétique qui décroît par application d'un paramètre extérieur tel que la pression, et qui devient du premier ordre avant de disparaître. Le changement d'ordre se fait à un point tricritique. D'autres mesures ont porté sur la transition supraconductrice qui se produit à l'intérieur de la phase ferromagnétique. La nature volumique de la supraconductivité a été confirmé et l'accent s'est porté sur son renforcement sous champ magnétique.; Cette thèse porte sur l'étude du composé supraconducteur ferromagnétique UGe2. La croissance de monocristaux de UGe2 a été réalisée dans un four tétra-arc par la technique du tirage Czochralski. Cette technique a également servie à l'obtention d'autres composés à base d'uranium, notament UCoGe et URu2Si2. Pour la première fois, la structure avec des ailes (wings) du diagramme de phase de UGe2 a été vérifiée expérimentalement. Cette observation est une conséquence d'une température de transition ferromagnétique qui décroît par application d'un paramètre extérieur tel que la pression, et qui devient du premier ordre avant de disparaître. Le changement d'ordre se fait à un point tricritique. D'autres mesures ont porté sur la transition supraconductrice qui se produit à l'intérieur de la phase ferromagnétique. La nature volumique de la supraconductivité a été confirmé et l'accent s'est porté sur son renforcement sous champ magnétique.

Cette thèse porte sur l'étude du composé supraconducteur ferromagnétique UGe2. La croissance de monocristaux de UGe2 a été réalisée dans un four tétra-arc par la technique du tirage Czochralski. Cette technique a également servie à l'obtention d'autres composés à base d'uranium, notament UCoGe et URu2Si2. Pour la première fois, la structure avec des ailes (wings) du diagramme de phase de UGe2 a été vérifiée expérimentalement. Cette observation est une conséquence d'une température de transition ferromagnétique qui décroît par application d'un paramètre extérieur tel que la pression, et qui devient du premier ordre avant de disparaître. Le changement d'ordre se fait à un point tricritique. D'autres mesures ont porté sur la transition supraconductrice qui se produit à l'intérieur de la phase ferromagnétique. La nature volumique de la supraconductivité a été confirmé et l'accent s'est porté sur son renforcement sous champ magnétique.; Cette thèse porte sur l'étude du composé supraconducteur ferromagnétique UGe2. La croissance de monocristaux de UGe2 a été réalisée dans un four tétra-arc par la technique du tirage Czochralski. Cette technique a également servie à l'obtention d'autres composés à base d'uranium, notament UCoGe et URu2Si2. Pour la première fois, la structure avec des ailes (wings) du diagramme de phase de UGe2 a été vérifiée expérimentalement. Cette observation est une conséquence d'une température de transition ferromagnétique qui décroît par application d'un paramètre extérieur tel que la pression, et qui devient du premier ordre avant de disparaître. Le changement d'ordre se fait à un point tricritique. D'autres mesures ont porté sur la transition supraconductrice qui se produit à l'intérieur de la phase ferromagnétique. La nature volumique de la supraconductivité a été confirmé et l'accent s'est porté sur son renforcement sous champ magnétique.

The aim of this study is to compare the magnetic behavior of URu2Si2 under uniaxial stress along the a axis with the behavior under hydrostatic pressure. Both are very similar, but uniaxial stress presents a critical stress σxa [0.33(5) GPa] that is smaller than the hydrostatic critical pressure px (0.5 GPa) where the ground state switches from a HO (hidden-order) to AF (antiferromagnetic) ground state. From these critical values and Larmor neutron diffraction, we conclude that the magnetic properties are governed by the shortest U-U distance in the plane (a lattice parameter). Under stress, the orthorhombic unit cell stays centered. A key point shown by this study is the presence of a threshold for the uniaxial stress along the a axis before the appearance of the large AF moment, which indicates no mixture of the order parameter between the HO ground state and the AF one as under hydrostatic pressure. The two most intense longitudinal magnetic excitations at Q0 =(1,0,0) and Q1 =(0.6,0,0) were measured in the HO state: the excitation at Q0 decreases in energy while the excitation at Q1 increases in energy with the uniaxial stress along the a axis. The decrease of the energy of the excitation at Q0 seems to indicate a critical energy-gap value of 1.2(1) meV at σxa. A similar value was derived from studies under hydrostatic pressure at px.

The excitation spectrum of URu 2 Si 2 in the superconducting state has been investigated by a precise neutron-scattering experiment. A weak but clear positive energy shift of ∼40 µeV has been detected in the superconducting phase (at T =400 mK) for the resonance at Q 0 = (1,0,0). For the resonance at Q 1 = (0.6,0,0), a two times smaller shift is observed but with an incertitude of the same order than this shift. This result is in agreement with our previous results indicating that the wave-vector Q 0 is connected with the order parameters of the hidden order and of superconductivity.