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Ronning, Filip

Publications and source records attributed to Ronning, Filip.

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Filling the holes in the CaFe 4 As 3 structure: Synthesis and magnetism of CaCo 5 As 3

Here, we investigate single crystals of CaCo 5 As 3 by means of single-crystal x-ray diffraction, microprobe, magnetic susceptibility, heat capacity, and pressure-dependent transport measurements. CaCo 5 As 3 shares the same structure of CaFe 4 As 3 with an additional Co atom filling a lattice vacancy and undergoes a magnetic transition at T M = 16 K associated with a frustrated magnetic order. CaCo 5 As 3 displays metallic behavior and its Sommerfeld coefficient (γ = 70 mJ/mol K 2 ) indicates a moderate enhancement of electron-electron correlations. Transport data under pressures to 2.5 GPa reveal a suppression of T M at a rate of - 0.008 K/GPa. First-principles electronic structure calculations show a complex three-dimensional band structure and magnetic moments that depend on the local environment at each Co site. Our results are compared with previous data on CaFe 4 As 3 and provide a scenario for a magnetically frustrated ground state in this family of compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Switching dynamics of the spin density wave in superconducting CeCoIn 5

The ordering wave vector Q of a spin density wave (SDW), stabilized within the superconducting state of CeCoIn 5 in a high magnetic field, has been shown to be hypersensitive to the direction of the field. Q can be switched from a nodal direction of the d-wave superconducting order parameter to a perpendicular node by rotating the in-plane magnetic field through the antinodal direction within a fraction of a degree. In this paper, we address the dynamics of the switching of Q. We use a free-energy functional based on the magnetization density, which describes the condensation of magnetic fluctuations of nodal quasiparticles, and show that the switching process includes closing of the SDW gap at one Q and then reopening the SDW gap at another Q perpendicular to the first one. The magnetic field couples to Q through the spin-orbit interaction. Our calculations show that the width of the hysteretic region of switching depends linearly on the deviation of magnetic field from the critical field associated with the SDW transition, consistent with our thermal conductivity measurements. Finally, the agreement between theory and experiment supports our scenario of the hypersensitivity of the Q phase on the direction of magnetic field, as well as the magnon condensation as the origin of the SDW phase in CeCoIn 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Inducing superconductivity in Weyl semimetal microstructures by selective ion sputtering

By introducing a superconducting gap in Weyl or Dirac semimetals, the superconducting state inherits the nontrivial topology of their electronic structure. As a result, Weyl superconductors are expected to host exotic phenomena, such as nonzero-momentum pairing due to their chiral node structure, or zero-energy Majorana modes at the surface. These are of fundamental interest to improve our understanding of correlated topological systems, and, moreover, practical applications in phase-coherent devices and quantum applications have been proposed. Proximity-induced superconductivity promises to allow these experiments on nonsuperconducting Weyl semimetals. Here, we show a new route to reliably fabricate superconducting microstructures from the nonsuperconducting Weyl semimetal NbAs under ion irradiation. Furthermore, the significant difference in the surface binding energy of Nb and As leads to a natural enrichment of Nb at the surface during ion milling, forming a superconducting surface layer (Tc ~ 3.5 K). Being formed from the target crystal itself, the ideal contact between the superconductor and the bulk may enable an effective gapping of the Weyl nodes in the bulk because of the proximity effect. Simple ion irradiation may thus serve as a powerful tool for the fabrication of topological quantum devices from monoarsenides, even on an industrial scale.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Resonances in the Field-Angle-Resolved Thermal Conductivity of CeCoIn 5

Here, the thermal conductivity measurement in a rotating magnetic field is a powerful probe of the structure of the superconducting energy gap. We present high-precision measurements of the low-temperature thermal conductivity in the unconventional heavy-fermion superconductor CeCoIn 5 , with the heat current J along the nodal [110] direction of its d x 2 –y 2 order parameter and the magnetic field up to 7 T rotating in the ab plane. In contrast to the smooth oscillations found previously for J∥[100], we observe a sharp resonancelike peak in the thermal conductivity when the magnetic field is also in the [110] direction, parallel to the heat current. We explain this peak qualitatively via a model of the heat transport in a d-wave superconductor. In addition, we observe two smaller but also very sharp peaks in the thermal conductivity for the field directions at angles Θ≈±33° with respect to J. The origin of the observed resonances at Θ≈±33° at present defies theoretical explanation. The challenge of uncovering their source will dictate exploring theoretically more complex models, which might include, e.g., fine details of the Fermi surface, Andreev bound vortex core states, a secondary superconducting order parameter, and the existence of gaps in spin and charge excitations.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Low temperature magnetic structure of CeRhIn 5 by neutron diffraction on absorption-optimized samples

Here, two aspects of the ambient pressure magnetic structure of heavy fermion material CeRhIn 5 have remained under some debate since its discovery: whether the structure is indeed an incommensurate helix or a spin density wave, and what is the precise magnitude of the ordered magnetic moment. By using a single crystal sample optimized for hot neutrons to minimize neutron absorption by Rh and In, here we report an ordered moment of $m=0.54(2)\,{{\mu}_{\text{B}}}$. In addition, by using spherical neutron polarimetry measurements on a similar single crystal sample, we have confirmed the helical nature of the magnetic structure, and identified a single chiral domain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Electronic in-plane symmetry breaking at field-tuned quantum criticality in CeRhIn5

Electronic nematicity appears in proximity to unconventional high-temperature superconductivity in the cuprates and iron-arsenides, yet whether they cooperate or compete is widely discussed. While many parallels are drawn between high-T c and heavy fermion superconductors, electronic nematicity was not believed to be an important aspect in their superconductivity. We have found evidence for a field-induced strong electronic in-plane symmetry breaking in the tetragonal heavy fermion superconductor CeRhIn 5 . At ambient pressure and zero field, it hosts an anti-ferromagnetic order (AFM) of nominally localized 4f electrons at TN=3.8K(1). Moderate pressure of 17kBar suppresses the AFM order and a dome of superconductivity appears around the quantum critical point. Similarly, a density-wave-like correlated phase appears centered around the field-induced AFM quantum critical point. In this phase, we have now observed electronic nematic behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Magnetic microstructure and magnetic properties of uniaxial itinerant ferromagnet Fe 3 GeTe 2

Here, magnetic force microscopy was used to observe the magnetic microstructure of Fe 3 GeTe 2 at 4 K on the (001) surface. The surface magnetic structure consists of a two-phase domain branching pattern that is characteristic for highly uniaxial magnets in the plane perpendicular to the magnetic easy axis. The average surface magnetic domain width D s = 1.3 μm determined from this pattern, in combination with intrinsic properties calculated from bulk magnetization data (the saturation magnetization M s = 376 emu/cm 3 and the uniaxial magnetocrystalline anisotropy constant K u = 1.46 × 10 7 erg/cm 3 ), was used to determine the following micromagnetic parameters for Fe 3 GeTe 2 from phenomenological models: the domain wall energy γ w = 4.7 erg/cm 2 , the domain wall thickness δ w = 2.5 nm, the exchange stiffness constant A ex = 0.95 × 10 –7 erg/cm, the exchange length l ex = 2.3 nm, and the critical single domain particle diameter d c = 470 nm.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Magnetic torque anomaly in the quantum limit of Weyl semimetals

Electrons in materials with linear dispersion behave as massless Weyl- or Dirac-quasiparticles, and continue to intrigue due to their close resemblance to elusive ultra-relativistic particles as well as their potential for future electronics. Yet the experimental signatures of Weyl-fermions are often subtle and indirect, in particular if they coexist with conventional, massive quasiparticles. Here we show a pronounced anomaly in the magnetic torque of the Weyl semimetal NbAs upon entering the quantum limit state in high magnetic fields. The torque changes sign in the quantum limit, signalling a reversal of the magnetic anisotropy that can be directly attributed to the topological nature of the Weyl electrons. Our results establish that anomalous quantum limit torque measurements provide a direct experimental method to identify and distinguish Weyl and Dirac systems.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Observation of Dirac-like semi-metallic phase in NdSb

The search of new topological phases of matter is one of the new directions in condensed matter physics. Recent experimental realizations of Dirac semimetal phases pave the way to look for other exotic phases of matter in real materials. In this paper, we present a systematic angle-resolved photoemission spectroscopy (ARPES) study of NdSb, a potential candidate for hosting a Dirac semi-metal phase. Our studies reveal two hole-like Fermi surface pockets present at the zone center ($\Gamma$ ) point as well as two elliptical electron-pockets present in the zone corner (X) point of the Brillouin zone (BZ). Interestingly, Dirac-like linearly dispersive states are observed about the zone corner (X) point in NdSb. Our first-principles calculations agree with the experimentally observed bands at the $\Gamma$ point. Moreover, the Dirac-like state observed in NdSb may be a novel correlated state, not yet predicted in calculations. Finally, our study opens a new direction to look for Dirac semi-metal states in other members of the rare earth monopnictide family.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Electronic Correlation and Magnetism in the Ferromagnetic Metal Fe 3 GeTe 2

Motivated by the search for design principles of rare-earth-free strong magnets, we present a study of electronic structure and magnetic properties of the ferromagnetic metal Fe3GeTe2 within local density approximation (LDA) of the density functional theory, and its combination with dynamical mean-field theory (DMFT). For comparison to these calculations, we have measured magnetic and thermodynamic properties as well as X-ray magnetic circular dichroism and the photoemission spectrum of single crystal Fe3GeTe2. We find that the experimentally determined Sommerfeld coefficient is enhanced by an order of magnitude with respect to the LDA value. This enhancement can be partially explained by LDA+DMFT. Additionally, the inclusion of dynamical electronic correlation effects provides the experimentally observed magnetic moments, and the spectral density is in better agreement with photoemission data. Lastly, these results establish the importance of electronic correlations in this ferromagnet.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Incommensurate to commensurate antiferromagnetism in CeRhAl 4 Si 2 : An Al 27 NMR study

27 Al nuclear magnetic resonance (NMR) experiments have been performed on a single crystal of CeRhAl 4 Si 2 , which is an antiferromagnetic Kondo-lattice compound with successive antiferromagnetic transitions of T N1 = 14 K and T N2 = 9 K at zero external field. In the paramagnetic state, the Knight shifts, quadrupolar frequency, and asymmetric parameter of electrical field gradient on the Al sites have been determined, which have local orthorhombic symmetry. The transferred hyperfine coupling constants are also determined. Here, analysis of the NMR spectra indicates that a commensurate antiferromagnetic structure exists below T N2 , but an incommensurate modulation of antiferromagnetic moments is present in the antiferromagnetic state between T N1 and T N2 . The spin-lattice relaxation rate suggests that the 4f electrons behave as local moments at temperatures above T N1 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electron-hole compensation effect between topologically trivial electrons and nontrivial holes in NbAs

In this study, via angular Shubnikov–de Haas (SdH) quantum oscillations measurements, we determine the Fermi surface topology of NbAs, a Weyl semimetal candidate. The SdH oscillations consist of two frequencies corresponding to two Fermi surface extrema: 20.8 T (α pocket) and 15.6 T (β pocket). The analysis, including a Landau fan plot, shows that the β pocket has a Berry phase of π and a small effective mass of ~ 0.033 m 0 , indicative of a nontrivial topology in momentum space, whereas the α pocket has a trivial Berry phase of 0 and a heavier effective mass of ~ 0.066 m 0 . From the effective mass and the β -pocket frequency, we determine that the Weyl node is 110.5 meV from the chemical potential. An electron-hole compensation effect is discussed in this system, and its impact on magnetotransport properties is addressed. Finally, the difference between NbAs and other monopnictide Weyl semimetals is also discussed.

36 MATERIALS SCIENCE↗