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Ronning, Filip

Publications and source records attributed to Ronning, Filip.

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Machine learning study of magnetism in uranium-based compounds

Actinide and lanthanide-based materials display exotic properties that originate from the presence of itinerant or localized f electrons and include unconventional superconductivity and magnetism, hidden order, and heavy-fermion behavior. Due to the strongly correlated nature of the 5f electrons, magnetic properties of these compounds depend sensitively on applied magnetic field and pressure, as well as on chemical doping. However, precise connection between the structure and magnetism in actinide-based materials is currently unclear. In this investigation, we established such structure-property links by assembling and mining two datasets that aggregate, respectively, the results of high-throughput density functional theory simulations and experimental measurements for the families of uranium- and neptunium-based binary compounds. Various regression algorithms were utilized to identify correlations among accessible attributes (features or descriptors) of the material systems and predict their cation magnetic moments and general forms of magnetic ordering. Descriptors representing compound structural parameters and cation f-subshell occupation numbers were identified as most important for accurate predictions. The best machine learning model developed employs the random forest regression algorithm. It can predict both spin and orbit moment size with root-mean-square error of 0.17 μ B and 0.19 μ B , respectively. Lastly, the random forest classification algorithm is used to predict the ordering (paramagnetic, ferromagnetic, and antiferromagnetic) of such systems with 76% accuracy.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interplay of the Spin Density Wave and a Possible Fulde-Ferrell-Larkin-Ovchinnikov State in CeCoIn 5 in Rotating Magnetic Field

The d-wave superconductor CeCoIn 5 has been proposed as a strong candidate for supporting the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state near the low-temperature boundary of its upper critical field. Neutron diffraction, however, finds spin-density-wave (SDW) order in this part of the phase diagram for field in the a-b plane, and evidence for the SDW disappears as the applied field is rotated toward the tetragonal c axis. It is important to understand the interplay between the SDW and a possible FFLO state in CeCoIn 5 , as the mere existence of an SDW does not necessarily exclude an FFLO state. In this work, based on a model constructed on the basis of available experiments, we show that an FFLO state competes with an SDW phase. The SDW state in CeCoIn 5 is stabilized when the field is directed close to the a-b plane. When the field is rotated toward the c axis, the FFLO state emerges, and the SDW phase disappears. In the FFLO state, the nodal planes with extra quasiparticles (where the superconducting order parameter is zero) are perpendicular to the field, and in the SDW phase, the quasiparticle density of states is reduced. We test this model prediction by measuring heat transported by normal quasiparticles in the superconducting state. As a function of field, we observe a reduction of thermal conductivity for field close to the a-b plane and an enhancement of thermal conductivity when field is close to the c axis, consistent with theoretical expectations. Our modeling and experiments, therefore, indicate the existence of the FFLO state when field is parallel to the c axis.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Anomalous Hall Effect in Kagome Ferrimagnet GdMn 6 Sn 6

We present magnetotransport data on the ferrimagnet GdMn 6 Sn 6 . From the temperature-dependent data we are able to extract a large intrinsic contribution to the anomalous Hall effect σ int xz ~ 32 Ω –1 cm –1 and σ int xz ~ 223 Ω –1 cm –1 , which is comparable to values found in other systems also containing kagome nets of transition metals. From our transport anisotropy, as well as our density functional theory calculations, we argue that the system is electronically best described as a three-dimensional system. Furthermore we show that reduced dimensionality is not a strong requirement for obtaining large Berry phase contributions to transport properties. In addition, the coexistence of rare-earth and transition-metal magnetism makes the hexagonal MgFe 6 Ge 6 structure type a promising system to tune the electronic and magnetic properties in future studies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Suppression of hybridization by Cd doping in CeCoIn 5

The interaction between superconductivity and magnetism in heavy fermion compounds has remained a challenge ever since the discovery of unconventional superconductivity in such systems. The heavy fermion Ce T In 5 ( T = Co,Rh,Ir) family offers a good platform to study the relationship between superconductivity and magnetism, whose ground state can be easily tuned by chemical doping, making the ambient-pressure experimental techniques like angle-resolved photoemission spectroscopy possible to address this old but engaging question. By Cd doping, the superconducting state of CeCoIn 5 can be smoothly tuned to an antiferromagnetic ground state, which can be reversed by applying pressure. In this study, we present an electronic structure study of CeCo ( In 0.85 Cd 0.15 ) 5 with an antiferromagnetic ground state and the results are compared with CeCoIn 5 . We found that the hybridization strength between the f electrons and conduction electrons in CeCoIn 5 has been suppressed by Cd doping, but still the Fermi surface includes a significant contribution from the Ce f electrons. Our results find that the electronic tuning effect by Cd doping arises from the change in the hybridization strength between the Ce f electrons and conduction electrons caused by the Cd dopant, and the subtle interplay between itinerancy and localization of the f electrons determines the ground state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Spatial control of heavy-fermion superconductivity in CeIrIn 5

While crystals of strongly correlated metals exhibit a diverse set of electronic ground states, few approaches exist for spatially modulating their properties. In this study, we demonstrate disorder-free control, on the micrometer scale, over the superconducting state in samples of the heavy-fermion superconductor CeIrIn 5 . We pattern crystals by focused ion beam milling to tailor the boundary conditions for the elastic deformation upon thermal contraction during cooling. The resulting nonuniform strain fields induce complex patterns of superconductivity, owing to the strong dependence of the transition temperature on the strength and direction of strain. These findings showcase a generic approach to manipulating electronic order on micrometer length scales in strongly correlated matter without compromising the cleanliness, stoichiometry, or mean free path.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Multicomponent fluctuation spectrum at the quantum critical point in CeCu 6- x Ag x

Quantum critical points (QCPs) are widely accepted as a source of a diverse set of collective quantum phases of matter. The basic nature of a QCP is manifested in the critical fluctuation spectrum which in turn is determined by the adjacent phases and associated order parameters. Here we show that the critical fluctuation spectrum of CeCu 5.8 Ag 0.2 can not be explained by fluctuations associated with a single wave vector. Interestingly, when the critical fluctuations at wave vectors corresponding to the incommensurate antiferromagnetic order adjacent to the QCP are separated they are found to be three dimensional and to obey the scaling behavior expected for long wavelength fluctuations near an itinerant antiferromagnetic QCP. Without this separation, E/T scaling with a fractional exponent is observed. Together these results demonstrate that a multicomponent fluctuation spectrum is a previously unexplored route to obtaining E/T scaling at a QCP.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Quantum Oscillations in Flux-Grown SmB 6 with Embedded Aluminum

SmB 6 is a candidate topological Kondo insulator that displays surface conduction at low temperatures. In this study, we perform torque magnetization measurements as a means to detect de Haas–van Alphen (dHvA) oscillations in SmB 6 crystals grown by aluminum flux. We find that dHvA oscillations occur in single crystals containing embedded aluminum, originating from the flux used to synthesize SmB 6 . Measurements on a sample with multiple, unconnected aluminum inclusions show that aluminum crystallizes in a preferred orientation within the SmB 6 cubic lattice. The presence of aluminum is confirmed through bulk susceptibility measurements, but does not show a signature in transport measurements. Lastly, we discuss the ramifications of our results.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Thermodynamic Signatures of Weyl Fermions in NbP

We present a high magnetic field study of NbP—a member of the monopnictide Weyl semimetal (WSM) family. While the monoarsenides (NbAs and TaAs) have topologically distinct left and right-handed Weyl fermi surfaces, NbP is argued to be “topologically trivial” due to the fact that all pairs of Weyl nodes are encompassed by a single Fermi surface. We use torque magnetometry to measure the magnetic response of NbP up to 60 tesla and uncover a Berry paramagnetic response, characteristic of the topological Weyl nodes, across the entire field range. At the quantum limit B* (≈32 T), τ/B experiences a change in slope when the chemical potential enters the last Landau level. Our calculations confirm that this magnetic response arises from band topology of the Weyl pocket, even though the Fermi surface encompasses both Weyl nodes at zero magnetic field. We also find that the magnetic field pulls the chemical potential to the chiral n = 0 Landau level in the quantum limit, providing a disorder-free way of accessing chiral Weyl fermions in systems that are “not quite” WSMs in zero magnetic field.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Comparing the anomalous Hall effect and the magneto-optical Kerr effect through antiferromagnetic phase transitions in Mn 3 Sn

In the non-collinear antiferromagnet Mn 3 Sn, we compare simultaneous measurements of the anomalous Hall effect (AHE) and the magneto-optical Kerr effect (MOKE) through two magnetic phase transitions: the high-temperature paramagnetic/antiferromagnetic (AF) phase transition at the Néel temperature ( T N ≈ 420 K) and a lower-temperature incommensurate magnetic ordering at T 1 ≈ 270 K. While both the AHE and MOKE are sensitive to the same underlying symmetries of the AF non-collinear spin order, we find that the transition temperatures measured by these two techniques unexpectedly differ by approximately 10 K. Moreover, the applied magnetic field at which the AF order reverses is significantly larger when measured by MOKE than when measured by AHE. Furthermore these results point to a difference between the bulk and surface magnetic properties of Mn 3 Sn.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Enhanced Hybridization Sets the Stage for Electronic Nematicity in CeRhIn 5

High magnetic fields induce a pronounced in-plane electronic anisotropy in the tetragonal antiferromagnetic metal CeRhIn 5 at H* ≳ 30 T for fields ≃20° off the $c$ axis. Here in this paper, we investigate the response of the underlying crystal lattice in magnetic fields to 45 T via high-resolution dilatometry. At low fields, a finite magnetic field component in the tetragonal $ab$ plane explicitly breaks the tetragonal (C 4 ) symmetry of the lattice revealing a finite nematic susceptibility. A modest a-axis expansion at H * hence marks the crossover to a fluctuating nematic phase with large nematic susceptibility. Magnetostriction quantum oscillations confirm a Fermi surface change at H * with the emergence of new orbits. By analyzing the field-induced change in the crystal-field ground state, we conclude that the in-plane Ce $f$ hybridization is enhanced at H * , in agreement with the in-plane lattice expansion. We argue that the nematic behavior observed in this prototypical heavy-fermion material is of electronic origin, and is driven by the hybridization between 4$f$ and conduction electrons which carries the $f$-electron anisotropy to the Fermi surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Multiple phases with intertwined magnetic and superconducting orders in Nd-doped CeCoIn 5

In this study, the thermal conductivity of heavy-fermion superconductor Ce 0.95 Nd 0.05 CoIn 5 was measured with a magnetic field rotating in the tetragonal a-b plane. The thermal conductivity exhibits a step within the high-field low-temperature (HFLT) phase, which exists above 8 Tesla within the superconducting state, when field is rotated through the antinodal [100] direction of the superconducting d-wave order parameter. This anomaly indicates the presence of a third order parameter within the HFLT phase, similar to that of pure CeCoIn 5 . Therefore, the HFLT state with triply intertwined orders, i.e., superconducting d-wave, magnetic spin-density wave (SDW), and a putative superconducting p-wave pair-density wave (PDW), is robust against 5% Nd doping. The second magnetic phase within the superconducting state below 8 Tesla, a low-field SDW, displays a hysteresis in thermal conductivity as a function of field magnitude, which is also revealing the existence of a third order consistent with a p-wave PDW. Lastly, given different origins and properties of the HFLT and low-field SDW states, the presence of three intertwined orders in both phases is remarkable and of far-reaching consequences.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Experimental and theoretical study of topology and electronic correlations in PuB 4

We synthesize single crystals of PuB 4 using an Al-flux technique. Single-crystal diffraction data provide structural parameters for first-principles density functional theory (DFT) calculations. By computing the density of states, the Z 2 topological invariant using the Wilson loop method, and the surface electronic structure from slab calculations, we find that PuB 4 is a nonmagnetic strong topological insulator with a band gap of 254 meV. Our magnetic susceptibility, heat capacity, and resistivity measurements are consistent with this analysis, albeit with a smaller gap of 35 meV. DFT plus dynamical mean-field theory calculations show that electronic correlations reduce the size of the band gap, and provide better agreement with the value determined by resistivity. Furthermore, these results demonstrate that PuB 4 is a promising actinide material to investigate the interplay of electronic correlations and nontrivial topology.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic phase dependence of the anomalous Hall effect in Mn 3 Sn single crystals

Thermodynamic and transport properties are reported on single crystals of the hexagonal antiferromagnet Mn 3 Sn grown by the Sn flux technique. Magnetization measurements reveal two magnetic phase transitions at T 1 = 275 K and T 2 = 200 K, below the antiferromagnetic phase transition at T N ≈ 420 K. The Hall conductivity in zero magnetic field is suppressed dramatically from 4.7 Ω -1 cm -1 to near zero below T 1 , coincident with the vanishing of the weak ferromagnetic moment. Finally, this illustrates that the large anomalous Hall effect arising from the Berry curvature can be switched on and off by a subtle change in the symmetry of the magnetic structure near room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Spectroscopic evidence for two-gap superconductivity in the quasi-1D chalcogenide Nb 2 Pd 0.81 S 5

Low-dimensional electronic systems with confined electronic wave functions have attracted interest due to their propensity toward novel quantum phases and their use in wide range of nanotechnologies. The newly discovered chalcogenide Nb 2 PdS 5 possesses a quasi-one-dimensional electronic structure and becomes superconducting. Here, we report spectroscopic evidence for two-band superconductivity, where soft point-contact spectroscopic measurements in the superconducting (SC) state reveal Andreev reflection in the differential conductance G. Multiple peaks in G are observed at 1.8 K and explained by the two-band Blonder–Tinkham–Klapwijk model with two gaps Δ 1 =0.61 meV and Δ 2 =1.20 meV. Finally, the progressive evolution of G with temperature and magnetic field corroborates the multiple nature of the SC gaps.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Synthesis and characterization of the heavy-fermion compound CePtAl 4 Ge 2

Here, we report the synthesis of the Ce-based quaternary compound CePtAl 4 Ge 2 that crystallizes in the trigonal structure (space group R 3¯m, 166) with unit cell parameters, a = 4.1995(5) Å, c = 31.851(7) Å, and γ = 120°. Powder X-ray diffraction and energy dispersive X-ray spectroscopy show that CePtAl 4 Ge 2 (LaPtAl 4 Ge 2 ) is in a single, homogeneous phase. Magnetic susceptibility, electrical resistivity, and heat capacity measurements of CePtAl 4 Ge 2 show that it exhibits antiferromagnetic behavior below 2.3 K. The magnetic susceptibility for the magnetic field applied perpendicular (χ ab ) and parallel (χ c ) to the crystalline c-axis is very anisotropic, and the susceptibility ratio (χ ab /χ c ) reaches a maximum value of 10, indicating that the spin easy axis is within the Ce plane. The entropy recovered at T N is consistent with the doublet ground state of the crystal field split J = 5/2 multiplet of Ce 3+ ions.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Emergent magnetic anisotropy in the cubic heavy-fermion metal CeIn3

Metals containing cerium exhibit a diverse range of fascinating phenomena including heavy fermion behavior, quantum criticality, and novel states of matter such as unconventional superconductivity. The cubic system CeIn3 has attracted significant attention as a structurally isotropic Kondo lattice material possessing the minimum required complexity to still reveal this rich physics. By using magnetic fields with strengths comparable to the crystal field energy scale, we illustrate a strong field-induced anisotropy as a consequence of non-spherically symmetric spin interactions in the prototypical heavy fermion material CeIn3. We demonstrate the importance of magnetic anisotropy in modeling f-electron materials when the orbital character of the 4f wavefunction changes (e.g., with pressure or composition). Additionally, magnetic fields are shown to tune the effective hybridization and exchange interactions potentially leading to new exotic field tuned effects in f-based materials.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Electronic in-plane symmetry breaking at field-tuned quantum criticality in CeRhIn 5

Electronic nematics are exotic states of matter where electronic interactions break a rotational symmetry of the underlying lattice, in analogy to the directional alignment without translational order in nematic liquid crystals. Intriguingly such phases appear in the copper- and iron-based superconductors, and their role in establishing high-temperature superconductivity remains an open question. Nematicity may take an active part, cooperating or competing with superconductivity, or may appear accidentally in such systems. Here we present experimental evidence for a phase of nematic character in the heavy fermion superconductor CeRhIn 5 . We observe a field-induced breaking of the electronic tetragonal symmetry of in the vicinity of an antiferromagnetic (AFM) quantum phase transition at H c ~50T. This phase appears in out-of-plane fields of H*~28T and is characterized by substantial in-plane resistivity anisotropy. The anisotropy can be aligned by a small in-plane field component, with no apparent connection to the underlying crystal structure. Furthermore no anomalies are observed in the magnetic torque, suggesting the absence of metamagnetic transitions in this field range. These observations are indicative of an electronic nematic character of the high field state in CeRhIn 5 . The appearance of nematic behavior in a phenotypical heavy fermion superconductor highlights the interrelation of nematicity and unconventional superconductivity, suggesting nematicity to be a commonality in such materials.

36 MATERIALS SCIENCE↗