arXiv subjects

E. D. Bauer

Publications and source records attributed to E. D. Bauer.

At least 91 records · Page 5Linked to original sources

arXiv2016

Directly probing spin dynamics in insulating antiferromagnets using ultrashort terahertz pulses

We investigate spin dynamics in the antiferromagnetic (AFM) multiferroic TbMnO3 using optical- pump, terahertz (THz)-probe spectroscopy. Photoexcitation results in a broadband THz transmission change, with an onset time of 25 ps at 6 K that becomes faster at higher temperatures. We attribute this time constant to spin-lattice thermalization. The excellent agreement between our measurements and previous ultrafast resonant x-ray diffraction measurements on the same material confirms that our THz pulse directly probes spin order. We suggest that this could be the case in general for insulating AFM materials, if the origin of the static absorption in the THz spectral range is magnetic.

cond-mat.str-el↗

arXiv2016

Probing 5f-state configurations in URu2Si2 with U L3-edge resonant x-ray emission spectroscopy

Resonant x-ray emission spectroscopy (RXES) was employed at the U L3 absorption edge and the La1 emission line to explore the 5f occupancy, nf, and the degree of 5f orbital delocalization in the hidden order compound URu2Si2. By comparing to suitable reference materials such as UF4, UCd11, and alpha-U, we conclude that the 5f orbital in URu2Si2 is at least partially delocalized with nf = 2.87 +/- 0.08, and does not change with temperature down to 10 K within the estimated error. These results place further constraints on theoretical explanations of the hidden order, especially those requiring a localized f2 ground state.

cond-mat.str-el↗

arXiv2016

Large magnetoresistance in the antiferromagnetic semi-metal NdSb

There has been considerable interest in topological semi-metals that exhibit extreme magnetoresistance (XMR). These have included materials lacking inversion symmetry such as TaAs, as well Dirac semi-metals such as Cd3As2. However, it was reported recently that LaSb and LaBi also exhibit XMR, even though the rock-salt structure of these materials has inversion symmetry, and the band-structure calculations do not show a Dirac dispersion in the bulk. Here, we present magnetoresistance and specific heat measurements on NdSb, which is isostructural with LaSb. NdSb has an antiferromagnetic groundstate, and in analogy with the lanthanum monopnictides, is expected to be a topologically non-trivial semi-metal. We show that NdSb has an XMR of 10^4 %, even within the AFM state, illustrating that XMR can occur independently of the absence of time reversal symmetry breaking in zero magnetic field. The persistence of XMR in a magnetic system offers promise of new functionality when combining topological matter with electronic correlations. We also find that in an applied magnetic field below the Neel temperature there is a first order transition, consistent with evidence from previous neutron scattering work.

cond-mat.str-el↗

arXiv2016

Anomalous electronic structure and magnetoresistance in TaAs$_2$

The resistance of a metal in a magnetic field can be very illuminating about its ground state. Some famous examples include the integer and fractional quantum Hall effects\cite{Klitzing-QHE,Tsui-FQHE}, Shubnikov-de Haas oscillations\cite{SdH}, and weak localization\cite{Lee-WL} \emph{et al}. In non-interacting metals the resistance typically increases upon the application of a magnetic field\cite{Pippard-MR}. In contrast, in some special circumstances metals, with anisotropic Fermi surfaces\cite{Kikugawa-PdCoO2LMR} or a so-called Weyl semimetal for instance\cite{Nielsen-ABJ,Son-ChirAnom}, may have negative magnetoresistance. Here we show that semimetallic TaAs$_2$ possesses a gigantic negative magnetoresistance ($-$98\% in a field of 3 T at low temperatures), with an unknown mechanism. Density functional calculations illustrate that TaAs$_2$ is a new topological semimetal [$\mathbb{Z}_2$ invariant (0;111)] without a Dirac dispersion. This demonstrates that the presence of negative magnetoresistance in non-magnetic semimetals cannot be uniquely attributed to the Adler-Bell-Jackiw anomaly of bulk Dirac/Weyl fermions. Our results also imply that the OsGe$_2$-type monoclinic dipnictides are likely a material basis where unconventional topological semimetals may be found.

cond-mat.str-el↗

arXiv2016

Vortexlike excitations in the heavy-fermion superconductor CeIrIn$_5$

We report a systematic study of temperature- and field-dependent charge ($\boldsymbolρ$) and entropy ($\mathbf{S}$) transport in the heavy-fermion superconductor CeIrIn$_5$. Its large positive thermopower $S_{xx}$ is typical of Ce-based Kondo lattice systems, and strong electronic correlations play an important role in enhancing the Nernst signal $S_{xy}$. By separating the off-diagonal Peltier coefficient $α_{xy}$ from $S_{xy}$, we find that $α_{xy}$ becomes positive and greatly enhanced at temperatures well above the bulk $T_c$. Compared with the non-magnetic analog LaIrIn$_5$, these results suggest vortexlike excitations in a precursor state to unconventional superconductivity in CeIrIn$_5$. This study sheds new light on the similarity of heavy-fermion and cuprate superconductors and on the possibility of states not characterized by the amplitude of an order parameter.

cond-mat.str-el↗

arXiv2016

Investigation of the physical properties of the Ce2MAl7Ge4 (M = Co, Ir, Ni, Pd) heavy fermion compounds

We report the synthesis, crystal structure and characterization by means of single crystal x-ray diffraction, neutron powder diffraction, magnetic, thermal and transport measurements of the new heavy fermion compounds Ce$_{2}$MAl$_{7}$Ge$_{4}$ (M = Co, Ir, Ni, Pd). These compounds crystallize in a noncentrosymmetic tetragonal space group P\={4}2$_{1}$m, consisting of layers of square nets of Ce atoms separated by Ge-Al and M-Al-Ge blocks. Ce$_{2}$CoAl$_{7}$Ge$_{4}$, Ce$_{2}$IrAl$_{7}$Ge$_{4}$ and Ce$_{2}$NiAl$_{7}$Ge$_{4}$ order magnetically behavior below $T_{M}=$ 1.8, 1.6, and 0.8 K, respectively. There is no evidence of magnetic ordering in Ce$_{2}$PdAl$_{7}$Ge$_{4}$ down to 0.4 K. The small amount of entropy released in the magnetic state of Ce$_{2}$MAl$_{7}$Ge$_{4}$ (M = Co, Ir, Ni) and the reduced specific heat jump at $T_M$ suggest a strong Kondo interaction in these materials. Ce$_{2}$PdAl$_{7}$Ge$_{4}$ shows non-Fermi liquid behavior, possibly due to the presence of a nearby quantum critical point.

cond-mat.str-el↗

arXiv2016

Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized $δ$-Pu

The effects on the local structure due to self-irradiation damage of Ga stabilized $δ$-Pu stored at cryogenic temperatures have been examined using extended x-ray absorption fine structure (EXAFS) experiments. Extensive damage, seen as a loss of local order, was evident after 72 days of storage below 15 K. The effect was observed from both the Pu and Ga sites, although less pronounced around Ga. Isochronal annealing was performed on this sample to study the annealing processes that occur between cryogenic and room temperature storage conditions, where damage is mostly reversed. Damage fractions at various points along the annealing curve have been determined using an amplitude-ratio method, standard EXAFS fitting, and a spherical crystallite model, and provide information complementary to previous electrical resistivity- and susceptibility-based isochronal annealing studies. The use of a spherical crystallite model accounts for the changes in EXAFS spectra using just two parameters, namely, the crystalline fraction and the particle radius. Together, these results are discussed in terms of changes to the local structure around Ga and Pu throughout the annealing process and highlight the unusual role of Ga in the behavior of the lowest temperature anneals.

cond-mat.mtrl-sci↗

arXiv2016

Optical spectroscopy and ultrafast pump-probe studies on the heavy fermion compound CePt$_2$In$_7$

We report optical spectroscopy and ultrafast pump-probe measurements on the antiferromagnetic heavy fermion compound CePt$_2$In$_7$, a member showing stronger two dimensionality than other compounds in CeIn$_3$-derived heavy-fermion family. We identify clear and typical hybridization spectral structures at low temperature from the two different spectroscopy probes. However, the strength and related energy scale of the hybridization are much weaker and smaller than that in the superconducting compounds CeCoIn$_5$ and CeIrIn$_5$. The features are more similar to observations on the antiferromagnetic compounds CeIn$_3$ and CeRhIn$_5$ in the same family. The results clearly indicate that the Kondo interaction and hybridizations exist in the antiferromagnetic compounds but with weaker strength.

cond-mat.str-el↗

arXiv2016

Comments on 'Phonon and magnetic structure in δ-plutonium from density-functional theory' by P. Söderlind et al

In their recent paper [Söderlind, P. et al., Sci. Rep. 5, 15958 (2015)], Söderlind et al. discuss two subjects from a theoretical point of view: the phonon spectra and the possible magnetic structure of δ-plutonium (Pu). Here, we comment on the second subject. Söderlind et al. compare the Pu magnetic form factor F(Q) calculated via density functional theory (DFT) with measurements of F(Q) by neutron spectroscopy [Janoschek, M. et al., Sci. Adv. 1, e1500188 (2015)]. In particular, this comparison does not consider a number of experimental facts established in the neutron spectroscopy study.

cond-mat.str-el↗

arXiv2016

Plutonium-Based Heavy-Fermion Systems

An effective mass of charge carriers that is significantly larger that the mass of a free electron develops at low temperatures in certain lanthanide- and actinide-based metals, including those formed with plutonium, due to strong electron-electron interactions. This heavy-fermion mass is reflected in a substantially enhanced electronic coefficient of specific heat $γ$, which for elemental Pu itself is much larger than that of normal metals. By our definition, there are twelve Pu-based heavy-fermion compounds, most discovered recently, whose basic properties are known and discussed. Relative to other examples, these Pu-based heavy-fermion systems are particularly complex due in part to the possible simultaneous presence of multiple, nearly degenerate 5f$^n$ configurations. This complexity poses significant opportunities as well as challenges, including understanding the origin of unconventional superconductivity in some of these materials.

cond-mat.str-el↗

arXiv2016

"Hard" crystalline lattice in the Weyl semimetal NbAs

We report the effect of hydrostatic pressure on the magnetotransport properties of the Weyl semimetal NbAs. Subtle changes can be seen in the $ρ_{xx}(T)$ profiles with pressure up to 2.31 GPa. The Fermi surfaces undergo an anisotropic evolution under pressure: the extremal areas slightly increase in the $\mathbf{k_x}$-$\mathbf{k_y}$ plane, but decrease in the $\mathbf{k_z}$-$\mathbf{k_y}$($\mathbf{k_x}$) plane. The topological features of the two pockets observed at atmospheric pressure, however, remain unchanged at 2.31 GPa. No superconductivity can be seen down to 0.3 K for all the pressures measured. By fitting the temperature dependence of specific heat to the Debye model, we obtain a small Sommerfeld coefficient $γ_0=$ 0.09(1) mJ/(mol$\cdot$K$^2$) and a large Debye temperature, $Θ_D=$ 450(9) K, confirming a "hard" crystalline lattice that is stable under pressure. We also studied the Kadowaki-Woods ratio of this low-carrier-density massless system, $R_{KW}=$ 3.2$\times 10^4$ $μΩ$ cm mol$^2$ K$^2$ J$^{-2}$. After accounting for the small carrier density in NbAs, this $R_{KW}$ indicates a suppressed transport scattering rate relative to other metals.

cond-mat.str-el↗

arXiv2016

Quantitative study of the f occupation in CeMIn5 and other cerium compounds with hard x-rays

We present bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) data of the Ce3d core levels and lifetime-reduced L edge x-ray absorption spectroscopy (XAS) in the partial fluorescence yield (PFY) mode of the CeMIn5 family with M = Co, Rh, and Ir. The HAXPES data are analyzed quantitatively with a combination of full multiplet and configuration interaction model which allows correcting for the strong plasmons in the CeMIn5 HAXPES data, and reliable weights wn of the different fn contributions in the ground state are determined. The CeMIn5 results are compared to HAXPES data of other heavy fermion compounds and a systematic decrease of the hybridization strength Veff from CePd3 to CeRh3B2 to CeRu2Si2 is observed, while it is smallest for the three CeMIn5 compounds. The f-occupation, however, increases in the same sequence and is close to one for the CeMIn5 family. The PFY-XAS data confirm an identical f-occupation in the three CeMIn5 compounds and a phenomenological fit to these PFY-XAS data combined with a configuration interaction model yields consistent results.

cond-mat.str-el↗

arXiv2015

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

Via angular Shubnikov-de Hass (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 $\sim$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 $\sim$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. A novel electron-hole compensation effect is discussed in this system, and its impact on magneto-transport properties is addressed. The difference between NbAs and other monopnictide Weyl semimetals is also discussed.

cond-mat.str-el↗

arXiv2015

Determination of spin and orbital magnetization in the ferromagnetic superconductor UCoGe

The magnetism in the ferromagnetic superconductor UCoGe has been studied using a combination of magnetic Compton scattering, bulk magnetization, X-ray magnetic circular dichroism and electronic structure calculations, in order to determine the spin and orbital moments. The experimentally observed total spin moment, $M_s$, was found to be -0.24 $\pm$ 0.05~$μ_B$ at 5~T. By comparison with the total moment of 0.16 $\pm$ 0.01~$μ_B$, the orbital moment, $M_l$, was determined to be 0.40 $\pm$ 0.05~$μ_B$. The U and Co spin moments were determined to be antiparallel. We find that the U 5\textit{f} electrons carry a spin moment of U$_s \approx$ -0.30~$μ_B$ and that there is a Co spin moment of Co$_s \approx$ 0.06~$μ_B$ induced via hybridization. The ratio U$_l/$U$_s$, of $-1.3 \pm 0.3$, shows the U moment to be itinerant. In order to ensure an accurate description of the properties of 5\textit{f} systems, and to provide a critical test of the theoretical approaches, it is clearly necessary to obtain experimental data for both the spin and orbital moments, rather than just the total magnetic moment. This can be achieved simply by measuring the spin moment with magnetic Compton scattering and comparing this to the total moment from bulk magnetization.

cond-mat.str-el↗

arXiv2015

The Valence-Fluctuating Ground State of Plutonium

A central issue in material science is to obtain understanding of the electronic correlations that control complex materials. Such electronic correlations frequently arise due to the competition of localized and itinerant electronic degrees of freedom. While the respective limits of well-localized or entirely itinerant ground states are well-understood, the intermediate regime that controls the functional properties of complex materials continues to challenge theoretical understanding. We have used neutron spectroscopy to investigate plutonium, which is a prototypical material at the brink between bonding and non-bonding configurations. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum-mechanical superposition of localized and itinerant electronic configurations as recently predicted by dynamical mean field theory. Our results not only resolve the long-standing controversy between experiment and theory on plutonium's magnetism, but also suggest an improved understanding of the effects of such electronic dichotomy in complex materials.

cond-mat.str-el↗

arXiv2015

Magnetotransport of single crystalline NbAs

We report transport measurement in zero and applied magnetic field on a single crystal of NbAs. Transverse and longitudinal magnetoresistance in the plane of this tetragonal structure does not saturate up to 9 T. In the transverse configuration ($H \parallel c$, $I \perp c$) it is 230,000 \% at 2 K. The Hall coefficient changes sign from hole-like at room temperature to electron-like below $\sim$ 150 K. The electron carrier density and mobility calculated at 2 K based on a single band approximation are 1.8 x 10$^{19}$ cm$^{-3}$ and 3.5 x 10$^{5}$ cm$^2$/Vs, respectively. These values are similar to reported values for TaAs and NbP, and further emphasize that this class of noncentrosymmetric, transition-metal monopnictides is a promising family to explore the properties of Weyl semimetals and the consequences of their novel electronic structure.

cond-mat.mtrl-sci↗

arXiv2015

Magnetic structure of the antiferromagnetic Kondo lattice compounds CeRhAl4Si2 and CeIrAl4Si2

We have investigated the magnetic ground state of the antiferromagnetic Kondo-lattice compounds CeMAl$_{4}$Si$_{2}$ (M = Rh, Ir) using neutron powder diffraction. Although both of these compounds show two magnetic transitions $T_{N1}$ and $T_{N2}$ in the bulk properties measurements, evidence for magnetic long-range order was only found below the lower transition $T_{N2}$. Analysis of the diffraction profiles reveals a commensurate antiferromagnetic structure with a propagation vector $\mathbf{k}$= (0, 0, 1/2). The magnetic moment in the ordered state of CeRhAl$_{4}$Si$_{2}$ and CeIrAl$_{4}$Si$_{2}$ were determined to be 1.14(2) and 1.41(3) $μ_{B}$/Ce, respectively, and are parallel to the crystallographic $c$-axis in agreement with magnetic susceptibility measurements.

cond-mat.str-el↗

arXiv2015

Magnetism and superconductivity in U_2Pt_xRh_(1-x)C_2

We report the phase diagram of the doping series U_2Pt_xRh_(1-x)C_2, studied through measurements of resistivity, specific heat and magnetic susceptibility. The Neel temperature of U_2RhC_2 of ~ 22 K is suppressed with increasing Pt content, reaching zero temperature close to x=0.7, where we observed signatures of increased quantum fluctuations. In addition, evidence is presented that the antiferromagnetic state undergoes a spin-reorientation transition upon application of an applied magnetic field. This transition shows non-monotonic behaviour as a function of x, peaking at around x=0.3. Superconductivity is observed for x>=0.9, with T_c increasing with increasing x. The reduction in T_c and increase in residual resistivity with decreasing Pt content is inconsistent with the extension of the Abrikosov-Gor'kov theory to unconventional superconductivity.

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