arXiv subjects

E. D. Bauer

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

At least 109 records · Page 6Linked to original sources

arXiv2015

Evidence for broken time-reversal symmetry in the superconducting phase of URu$_2$Si$_2$

Recent experimental and theoretical interest in the superconducting phase of the heavy fermion material URu$_2$Si$_2$ has led to a number of proposals in which the superconducting order parameter breaks time-reversal symmetry (TRS). In this study we measured polar Kerr effect (PKE) as a function of temperature for several high-quality single crystals of URu$_2$Si$_2$. We find an onset of PKE below the superconducting transition that is consistent with a TRS-breaking order parameter. This effect appears to be independent of an additional, possibly extrinsic, PKE generated above the hidden order transition at $T_{HO}=17.5$ K, and contains structure below $T_c$ suggestive of additional physics within the superconducting state.

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arXiv2014

Chemical pressure tuning of URu$_2$Si$_2$ via isoelectronic substitution of Ru with Fe

We have used specific heat and neutron diffraction measurements on single crystals of URu$_{2-x}$Fe$_x$Si$_2$ for Fe concentrations $x$ $\leq$ 0.7 to establish that chemical substitution of Ru with Fe acts as "chemical pressure" $P_{ch}$ as previously proposed by Kanchanavatee et al. [Phys. Rev. B {\bf 84}, 245122 (2011)] based on bulk measurements on polycrystalline samples. Notably, neutron diffraction reveals a sharp increase of the uranium magnetic moment at $x=0.1$, reminiscent of the behavior at the "hidden order" (HO) to large moment antiferromagnetic (LMAFM) phase transition observed at a pressure $P_x\approx$ 0.5-0.7~GPa in URu$_2$Si$_2$. Using the unit cell volume determined from our measurements and an isothermal compressibility $κ_{T} = 5.2 \times 10^{-3}$ GPa$^{-1}$ for URu$_2$Si$_2$, we determine the chemical pressure $P_{ch}$ in URu$_{2-x}$Fe$_x$Si$_2$ as a function of $x$. The resulting temperature $T$-chemical pressure $P_{ch}$ phase diagram for URu$_{2-x}$Fe$_x$Si$_2$ is in agreement with the established temperature $T$-external pressure $P$ phase diagram of URu$_2$Si$_2$.

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arXiv2014

Investigation of the physical properties of the tetragonal CeMAl4Si2 (M = Rh, Ir, Pt) compounds

The synthesis, crystal structure, and physical properties studied by means of x-ray diffraction, magnetic, thermal and transport measurements of CeMAl$_{4}$Si$_{2}$ (M = Rh, Ir, Pt) are reported, along with the electronic structure calculations for LaMAl$_{4}$Si$_{2}$ (M = Rh, Ir, Pt). These materials adopt a tetragonal crystal structure (space group P4/mmm) comprised of BaAl$_4$ blocks, separated by MAl$_2$ units, stacked along the $c$-axis. Both CeRhAl$_{4}$Si$_{2}$ and CeIrAl$_{4}$Si$_{2}$ order antiferromagnetically below $T_{N1}$=14 and 16 K, respectively, and undergo a second antiferromagnetic transitition at lower temperature ($T_{N2}$=9 and 14 K, respectively). CePtAl$_{4}$Si$_{2}$ orders ferromagnetically below $T_C$ =3 K with an ordered moment of $μ_{sat}$=0.8 $μ_{B}$ for a magnetic field applied perpendicular to the $c$-axis. Electronic structure calculations reveal quasi-2D character of the Fermi surface.

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arXiv2014

Quantum Critical Fluctuations in the Heavy fermion compound Ce(Ni$_{0.935}$Pd$_{0.065}$)$_2$Ge$_2$

Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experiments were performed on a single crystal of the heavy fermion compound Ce(Ni$_{0.935}$Pd$_{0.065}$)$_2$Ge$_2$ in order to study the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for $T \leq$ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP ($ρ(T) \sim T^{3/2}$ and $γ(T) \sim γ_0 - b T^{1/2}$). However, for 2 $\leq T \leq$ 10 K, the susceptibility and specific heat vary as $log T$ and the resistivity varies linearly with temperature. Furthermore, despite the fact that the resistivity and specific heat exhibit the non-Fermi liquid behavior expected at a QCP, the correlation length, correlation time, and staggered susceptibility of the spin fluctuations remain finite at low temperature. We suggest that these deviations from the divergent behavior expected for a QCP may result from alloy disorder.

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arXiv2014

Chirality density wave of the 'hidden order' phase in URu$_2$Si$_2$

A second-order phase transition is associated with emergence of an "order parameter" and a spontaneous symmetry breaking. For the heavy fermion superconductor URu$_2$Si$_2$, the symmetry of the order parameter associated with its ordered phase below 17.5 K has remained ambiguous despite 30 years of research, and hence is called "hidden order" (HO). Here we use polarization resolved Raman spectroscopy to specify the symmetry of the low energy excitations above and below the HO transition. These excitations involve transitions between interacting heavy uranium 5f orbitals, responsible for the broken symmetry in the HO phase. From the symmetry analysis of the collective mode, we determine that the HO parameter breaks local vertical and diagonal reflection symmetries at the uranium sites, resulting in crystal field states with distinct chiral properties, which order to a commensurate chirality density wave ground state.

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arXiv2014

Short-range magnetic correlations in the highly-correlated electron compound CeCu$_{4}Ga

We present experimental results for the heavy-electron compound CeCu$_{4}$Ga which show that it possesses short-range magnetic correlations down to a temperature of $T = 0.1$ K. Our neutron scattering data show no evidence of long-range magnetic order occurring despite a peak in the specific heat at $T^{*} =1.2$ K. Rather, magnetic diffuse scattering occurs which corresponds to short-range magnetic correlations occurring across two unit cells. The specific heat remains large as $T\sim0$ K resulting in a Sommerfeld coefficient of $γ_{0} = 1.44(2)$ J/mol-K$^{2}$, and, below $T^{*}$, the resistivity follows $T^{2}$ behavior and the ac magnetic susceptibility becomes temperature independent. A magnetic peak centered at an energy transfer of $E_{\rm{c}}=0.24(1)$ meV is seen in inelastic neutron scattering data which shifts to higher energies and broadens under a magnetic field. We discuss the coexistence of large specific heat, magnetic fluctuations, and short-range magnetic correlations at low temperatures and compare our results to those for materials possessing spin-liquid behavior.

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arXiv2014

Avoided Valence Transition in a Plutonium Superconductor

Some of the most remarkable phenomena---and greatest theoretical challenges---in condensed matter physics arise when $d$ or $f$ electrons are neither fully localized around their host nuclei, nor fully itinerant. This localized/itinerant "duality" underlies the correlated electronic states of the high-$T_c$ cuprate superconductors and the heavy-fermion intermetallics, and is nowhere more apparent than in the $5f$ valence electrons of plutonium. Here we report the full set of symmetry-resolved elastic moduli of $PuCoGa_5$---the highest $T_c$ superconductor of the heavy fermions ($T_c$=18.5 K)---and find that the bulk modulus softens anomalously over a wide range in temperature above $T_c$. Because the bulk modulus is known to couple strongly to the valence state, we propose that plutonium valence fluctuations drive this elastic softening. This elastic softening is observed to disappear when the superconducting gap opens at $T_c$, suggesting that plutonium valence fluctuations have a strong footprint on the Fermi surface, and that $PuCoGa_5$ avoids a valence-transition by entering the superconducting state. These measurements provide direct evidence of a valence instability in a plutonium compound, and suggest that the unusually high-$T_c$ in this system is driven by valence fluctuations.

cond-mat.supr-con↗

arXiv2014

Two-channel point-contact tunneling theory of superconductors

We introduce a two-channel tunneling model to generalize the widely used BTK theory of point-contact conductance between a normal metal contact and superconductor. Tunneling of electrons can occur via localized surface states or directly, resulting in a Fano resonance in the differential conductance $G=dI/dV$. We present an analysis of $G$ within the two-channel model when applied to soft point-contacts between normal metallic silver particles and prototypical heavy-fermion superconductors CeCoIn$_5$ and CeRhIn$_5$ at high pressures. In the normal state the Fano line shape of the measured $G$ is well described by a model with two tunneling channels and a large temperature-independent background conductance. In the superconducting state a strongly suppressed Andreev reflection signal is explained by the presence of the background conductance. We report Andreev signal in CeCoIn$_5$ consistent with standard $d_{x^2-y^2}$-wave pairing, assuming an equal mixture of tunneling into [100] and [110] crystallographic interfaces. Whereas in CeRhIn$_5$ at 1.8 and 2.0 GPa the signal is described by a $d_{x^2-y^2}$-wave gap with reduced nodal region, i.e., increased slope of the gap opening on the Fermi surface. A possibility is that the shape of the high-pressure Andreev signal is affected by the proximity of a line of quantum critical points that extends from 1.75 to 2.3 GPa, which is not accounted for in our description of the heavy-fermion superconductor.

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arXiv2014

The magnitude of the magnetic exchange interaction in the heavy fermion antiferromagnet CeRhIn$_5$

We have used high-resolution neutron spectroscopy experiments to determine the complete spin wave spectrum of the heavy fermion antiferromagnet CeRhIn$_5$. The spin wave dispersion can be quantitatively reproduced with a simple $J_1$-$J_2$ model that also naturally explains the magnetic spin-spiral ground state of CeRhIn$_5$ and yields a dominant in-plane nearest-neighbor magnetic exchange constant $J_0$ = 0.74 meV. Our results pave the way to a quantitative understanding of the rich low-temperature phase diagram of the prominent Ce$T$In$_5$ ($T$ = Co, Rh, Ir) class of heavy fermion materials.

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arXiv2014

Complex magnetism and strong electronic correlations in Ce2PdGe3

Single crystal x-ray diffraction, magnetic susceptibility, heat capacity, and electrical resistivity measurements are reported for the new tetragonal compound Ce$_{2}$PdGe$_{3}$, which forms in the space group $P4_{2}/mmc$ (\#131) $-$ a relative of the $α$-ThSi$_2$-type structure. Measurements reveal a two-part antiferromagnetic phase transition at $T_{\rm{N,1}}$ $=$ 10.7 K and $T_{\rm{N,2}}$ $=$ 9.6 K and subsequent ferromagnetic ordering near $T_{\rm{C}}$ $\approx$ 2.25 K. The ordered ground state emerges from a lattice of Ce ions that are hybridized with the conduction electrons, as revealed by the enhanced electronic coefficient of the specific heat $γ$ $\approx$ 50 mJ/mol-Ce-K$^2$ (extrapolated to $T$ $=$ 0 for $T$ $<$ $T_{\rm{C}}$). Electronic structure calculations suggest that there is significant f-electron weight in the density of states near the Fermi energy and that the Fermi surface includes sheets with distinct nesting vectors. Chemical/structural disorder also plays an important role, as evidenced by results from single crystal x-ray diffraction, the width of the peaks in the heat capacity at $T_{\rm{N}}$ and $T_{\rm{C}}$, and the small residual resistivity ratio RRR $=$ $ρ_{\rm{300K}}$/$ρ_{\rm{0}}$ $=$ 1.8.

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arXiv2014

Correlation between the phase diagram and the crystal-field wave functions of CeRh(1-x)Ir(x)In5

The 4f crystal-electric field ground state wave functions of CeRh(1-x)Ir(x)In5 have been determined by linear polarization dependent soft x-ray absorption spectroscopy (XAS) at T = 8 K. We demonstrate that these ground state wave functions correlate with the phase diagram of the substitution series, which covers long-range antiferromagnetic order, unconventional superconductivity, and coexistence of these two states. We discuss how certain wave functions could affect magnetic order and anisotropic hybridization, facilitating the formation of a superconducting ground state at low temperatures.

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arXiv2014

Evidence for spin-triplet superconductivity in U$_2$PtC$_2$ from $^{195}$Pt NMR

Nuclear magnetic resonance (NMR) measurements on the $^{195}$Pt nucleus in an aligned powder of the moderately heavy-fermion material U2PtC2 are consistent with spin-triplet pairing in its superconducting state. Across the superconducting transition temperature and to much lower temperatures, the NMR Knight shift is temperature independent for field both parallel and perpendicular to the tetragonal c-axis, expected for triplet equal-spin pairing superconductivity. The NMR spin-lattice relaxation rate 1/T$_1$, in the normal state, exhibits characteristics of ferromagnetic fluctuations, compatible with an enhanced Wilson ratio. In the superconducting state, 1/T$_1$ follows a power law with temperature without a coherence peak giving additional support that U$_2$PtC$_2$ is an unconventional superconductor. Bulk measurements of the AC-susceptibility and resistivity indicate that the upper critical field exceeds the Pauli limiting field for spin-singlet pairing and is near the orbital limiting field, an additional indication for spin-triplet pairing.

cond-mat.supr-con↗

arXiv2014

Disorder in a Quantum Critical Superconductor

In four classes of materials, the layered copper-oxides, organics, iron-pnictides and heavy-fermion compounds, an unconventional superconducting state emerges as a magnetic transition is tuned toward absolute zero temperature, that is, toward a magnetic quantum-critical point (QCP). In most materials, the QCP is accessed by chemical substitutions or applied pressure. CeCoIn5 is one of the few materials that are born as a quantum-critical superconductor and, therefore, offers the opportunity to explore the consequences of chemical disorder. Cadmium-doped crystals of CeCoIn5 are a particularly interesting case where Cd substitution induces long-range magnetic order, as in Zn-doped copper-oxides. Applied pressure globally supresses the Cd-induced magnetic order and restores bulk superconductivity. Here we show, however, that local magnetic correlations, whose spatial extent decreases with applied pressure, persist at the extrapolated QCP. The residual droplets of impurity-induced magnetic moments prevent the reappearance of conventional signatures of quantum criticality, but induce a heterogeneous electronic state. These discoveries show that spin droplets can be a source of electronic heterogeneity in classes of strongly correlated electron systems and emphasize the need for caution when interpreting the effects of tuning a correlated system by chemical substitution.

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arXiv2014

Magnetic Order in the 2D Heavy-Fermion System CePt2In7 studied by muSR

The low-temperature microscopic magnetic properties of the quasi-2D heavyfermion compound, CePt2In7 are investigated by using a positive muon-spin rotation and relaxation (?muSR) technique. Clear evidence for the formation of a commensurate antiferromagnetic order below TN=5.40 K is presented. The magnetic order parameter is shown to fit well to a modified BSC gap-energy function in a strong-coupling scenario.

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arXiv2014

Evidence for a nematic component to the Hidden Order parameter in URu2Si2 from differential elastoresistance measurements

Measurements of the differential elastoresistance of URu$_2$Si$_2$ reveal that the fluctuations associated with the 17 K Hidden Order phase transition have a nematic component. Approaching the "Hidden Order" phase transition from above, the nematic susceptibility abruptly changes sign, indicating that while the Hidden Order phase has a nematic component, it breaks additional symmetries.

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arXiv2014

Microscopic Properties of the Heavy-Fermion Superconductor PuCoIn$_5$ Explored by Nuclear Quadrupole Resonance

We report $^{115}$In nuclear quadrupolar resonance (NQR) measurements on the heavy-fermion superconductor PuCoIn$_5$, in the temperature range $0.29{\rm K}\leq T\leq 75{\rm K}$. The NQR parameters for the two crystallographically inequivalent In sites are determined, and their temperature dependence is investigated. A linear shift of the quadrupolar frequency with lowering temperature below the critical value $T_c$ is revealed, in agreement with the prediction for composite pairing. The nuclear spin-lattice relaxation rate $T_1^{-1}(T)$ clearly signals a superconducting (SC) phase transition at $T_c\simeq 2.3$K, with strong spin fluctuations, mostly in-plane, dominating the relaxation process in the normal state near to $T_c$. Analysis of the $T_1^{-1}$ data in the SC state suggests that PuCoIn$_5$ is a strong-coupling $d$-wave superconductor.

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arXiv2014

Q-dependence of the spin fluctuations in the intermediate valence compound CePd3

We report inelastic neutron scattering experiments on a single crystal of the intermediate valence compound CePd3. At 300 K the magnetic scattering is quasielastic, with halfwidth of 23 meV, and is independent of momentum transfer Q. At low temperature, the Q-averaged magnetic spectrum is inelastic, exhibiting a broad peak centered near Emax = 55 meV. These results, together with the temperature dependence of the susceptibility, 4f occupation number, and specific heat, can be fit by the Kondo/Anderson impurity model. The low temperature scattering near Emax, however, shows significant variations with Q, reflecting the coherence of the 4f lattice. The intensity is maximal at (1/2, 1/2,0), intermediate at (1/2,0,0) and (0,0,0), and weak at (1/2,1/2,1/2). We discuss this Q-dependence in terms of current ideas about coherence in heavy fermion systems.

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arXiv2013

CeIrIn5: Superconductivity on a Magnetic Instability

We report the doping-induced antiferromagnetic state and Fermi liquid state that are connected by a superconducting region in a series of CeIrIn$_{5-x}$Hg$_x$, CeIrIn$_{5-x}$Sn$_x$ and CeIr$_{1-x}$Pt$_x$In$_5$ single crystals. Measurements of the specific heat $C(T)$ and electrical resistivity $ρ(T)$ demonstrate that hole doping via Hg/In substitution gives rise to an antiferromagnetic ground state, but substitutions of In by Sn or Ir by Pt (electron doping) favor a paramagnetic Fermi liquid state. A cone-like non-Fermi liquid region is observed near CeIrIn$_5$, showing a diverging effective mass on the slightly Hg-doped side. The obtained temperature-doping phase diagram suggests that CeIrIn$_5$ is in proximity to an antiferromagnetic quantum critical point, and heavy fermion superconductivity in this compound is mediated by magnetic quantum fluctuations rather than by valence fluctuations.

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