arXiv subjects

F. Steglich

Publications and source records attributed to F. Steglich.

At least 37 records · Page 2Linked to original sources

arXiv2015

Towards ferromagnetic quantum criticality in FeGa3-xGex: 71Ga NQR as a zero field microscopic probe

71Ga NQR, magnetization and specific heat measurements have been performed on polycrystalline Ge doped FeGa3 samples. A crossover from an insulator to a correlated local moment metal in the low doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, xC=0.15, (1/T1T) exhibits a pronounced T4/3 power law over two orders of magnitude in temperature which indicates 3D quantum critical ferromagnetic fluctuations. Furthermore, for the ordered x = 0.2 sample (TC = 6 K) 1/T1T could be fitted well in the frame of Moriya's SCR theory for weakly FM systems with 1/T1T proportional to chi. In contrast to this, the low doping regime nicely displays local moment behavior where 1/T1T proportional to chi2 is valid. For temperature tending to zero the Sommerfeld ratio = (C/T) is enhanced (70 mJ/mole-K2 for x = 0.1) which indicates the formation of heavy 3d-electrons.

cond-mat.str-el↗

arXiv2015

Weak Interband-Coupling Superconductivity in the Filled Skutterudite LaPt4Ge12

The superconducting pairing state of LaPt$_{4}$Ge$_{12}$ is studied by measuring the magnetic penetration depth $λ(T,B)$ and the superfluid density $ρ_s(T)$ using a tunnel-diode-oscillator (TDO)-based method and by transverse field muon-spin rotation ($μ$SR) spectroscopy. $λ(T)$ follows an exponential-type temperature dependence at $T\ll T_{c}$, but its zero-temperature value $λ(0)$ increases linearly with magnetic field. Detailed analyses demonstrate that both $λ(T)$ and the corresponding $ρ_{s}(T)$, measured in the Meissner state by the TDO method are well described by a two-gap $γ$ model with gap sizes of $Δ_1(0)=1.31k_{B}T_c$ and $Δ_2(0)=1.80k_{B}T_c$ and a very weak interband coupling. In contrast, $ρ_s(T)$, derived from the $μ\rm{SR}$ data taken in a small field, can be fitted by a single-gap BCS model with a gap close to $Δ_2(0)$. We conclude that LaPt$_{4}$Ge$_{12}$ is a marginal two-gap superconductor and the small gap $Δ_1$ seems to be destroyed by a small magnetic field. In comparison, in PrPt$_4$Ge$_{12}$ the $4f$-electrons may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity.

cond-mat.supr-con↗

arXiv2015

Kondo Destruction in Heavy Fermion Quantum Criticality and the Photoemission Spectrum of YbRh$_2$Si$_2$

Heavy fermion metals provide a prototype setting to study quantum criticality. Experimentally, quantum critical points have been identified and studied in a growing list of heavy fermion compounds. Theoretically, Kondo destruction has provided a means to characterize a class of unconventional quantum critical points that goes beyond the Landau framework of order-parameter fluctuations. Among the prominent evidence for such local quantum criticality have been measurements in YbRh$_2$Si$_2$. A rapid crossover is observed at finite temperatures in the isothermal field dependence of the Hall coefficient and other transport and thermodynamic quantities, which specifies a $T^*(B)$ line in the temperature ($T$)-magnetic field ($B$) phase diagram. Here, we discuss what happens when temperature is raised, by analyzing the ratio of the crossover width to the crossover position. With this ratio approaching unity at $T \gtrsim 0.5$ K, YbRh$_2$Si$_2$ at zero magnetic field belongs to the quantum-critical fluctuation regime, where the single-particle spectral function has significant spectral weight at both the small and large Fermi surfaces. This implies that, in this temperature range, any measurements sensitive to the Fermi surface will also see a significant spectral weight at the large Fermi surface. The angle-resolved photoemission spectroscopy (ARPES) experiments recently reported for YbRh$_2$Si$_2$ at $T>1$ K are consistent with this expectation, and therefore support the association of the $T^*(B)$ line with the physics of Kondo destruction.

cond-mat.str-el↗

arXiv2014

Contiguous 3d and 4f magnetism: towards strongly correlated 3d electrons in YbFe2Al10

We present magnetization, specific heat, and 27Al NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T) divergence of the low-T specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard X-ray photo emission spectroscopy (HAXPES) study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, Yb3+ carries a full and stable moment, and Fe carries a moment of about 3.1 mB. The enhanced value of the Sommerfeld Wilson ratio and the dynamic scaling of spin-lattice relaxation rate divided by T [27(1/T1T)] with static susceptibility suggests admixed ferromagnetic correlations. 27(1/T1T) simultaneously tracks the valence fluctuations from the 4f -Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3d moments at low temperature, the latter evolve out of an Yb 4f admixed conduction band.

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arXiv2014

Thermodynamic evidence for valley-dependent density of states in bulk bismuth

Electron-like carriers in bismuth are described by the Dirac Hamiltonian, with a band mass becoming a thousandth of the bare electron mass along one crystalline axis. The existence of three anisotropic valleys offers electrons an additional degree of freedom, a subject of recent attention. Here, we map the Landau spectrum by angle-resolved magnetostriction, and quantify the carrier number in each valley: while the electron valleys keep identical spectra, they substantially differ in their density of states at the Fermi level. Thus, the electron fluid does not keep the rotational symmetry of the lattice at low temperature and high magnetic field, even in the absence of internal strain. This effect, reminiscent of the Coulomb pseudo-gap in localized electronic states, affects only electrons in the immediate vicinity of the Fermi level. It presents the most striking departure from the non-interacting picture of electrons in bulk bismuth.

cond-mat.str-el↗

arXiv2013

BCS-like superconductivity in the noncentrosymmetric compounds Nb$_x$Re$_{1-x}$

We present research on the superconducting properties of Nb$_{x}$Re$_{1-x}$ ($x$ = 0.13-0.38) obtained by measuring the electrical resistivity $ρ(T)$, magnetic susceptibility $χ(T)$, specific heat $C_P(T)$, and London penetration depth $Δλ(T)$. It is found that the superconducting transition temperature $T_c$ decreases monotonically with an increase of $x$. The upper critical field $B_{c2}(T)$ for various $x$ can be nicely scaled by its corresponding $T_c$. The electronic specific heat $C_e(T)/T$, penetration depth $Δλ(T)$, and superfluid density $ρ_{s}(T)$ demonstrate exponential behavior at low temperatures and can be well fitted by a one-gap BCS model. The residual Sommerfeld coefficient $γ_0(B)$ in the superconducting state follows a linear field dependence. All these properties suggest an \emph{s}-wave BCS-type of superconductivity with a very large $B_{c2}(0)$ for Nb$_{x}$Re$_{1-x}$ (0.13 $\leq x \leq$ 0.38).

cond-mat.supr-con↗

arXiv2013

Specific heat of segmented Heisenberg quantum spin chains in (Yb_{1-x}Lu_x)_4As_3

We report low-temperature specific heat, $C(T)$, measurements on (Yb$_{1-x}$Lu$_x$)$_4$As$_3$ with $x=0.01$ and $x=0.03$, where nonmagnetic Lu atoms are randomly distributed on antiferromagnetic $S=1/2$ Heisenberg chains with $J/k_{\mathrm B}=28$ K. The observed reduction of $C$ below 15 K with increasing $x$ is accurately described by quantum transfer matrix simulations without any adjustable parameter, implying that the system is an excellent experimental realization of segmented quantum spin chains. Finite-size effects consistent with conformal-field theory predictions are leading to the formation of an effective low-energy gap. The size of the gap increases with Lu content and accounts for the impurity driven reduction of the specific heat. For both concentrations our results verify experimentally the low temperature scaling behavior established theoretically and also confirm the value of $J$ determined from pure Yb$_4$As$_3$.

cond-mat.str-el↗

arXiv2013

Hybridization gap and Fano resonance in SmB${_6}$

We present results of Scanning Tunneling Microscopy and Spectroscopy (STS) measurements on the "Kondo insulator" SmB$_6$. The vast majority of surface areas investigated was reconstructed but, infrequently, also patches of varying size of non-reconstructed, Sm- or B-terminated surfaces were found. On the smallest patches, clear indications for the hybridization gap and inter-multiplet transitions were observed. On non-reconstructed surface areas large enough for coherent co-tunneling we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure.

cond-mat.str-el↗

arXiv2013

Multiple quantum phase transitions in a heavy fermion antiferromagnet

We report measurements of magnetic quantum oscillations and specific heat at low temperatures across a field-induced antiferromagnetic quantum critical point (QCP)(B_{c0}\approx50T) of the heavy-fermion metal CeRhIn_5. A sharp magnetic-field induced Fermi surface reconstruction is observed inside the antiferromagnetic phase. Our results demonstrate multiple classes of QCPs in the field-pressure phase diagram of this heavy-fermion metal, pointing to a universal description of QCPs. They also suggest that robust superconductivity is promoted by unconventional quantum criticality of a fluctuating Fermi surface.

cond-mat.str-el↗

arXiv2013

Thermal and Electrical Transport across a Magnetic Quantum Critical Point

A quantum critical point (QCP) arises at a continuous transition between competing phases at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau's Fermi liquid description, the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations which possess the quantum numbers of the bare electrons. Here we report measurements of thermal and electrical transport across the field-induced magnetic QCP in the heavy-fermion compound YbRh$_2$Si$_2$. We show that the ratio of the thermal to electrical conductivities at the zero-temperature limit obeys the Wiedemann-Franz (WF) law above the critical field, $B_c$. This is also expected at $B < B_c$, where weak antiferromagnetic order and a Fermi liquid phase form below 0.07 K ($B = 0$). However, at the critical field the low-temperature electrical conductivity suggests a non-Fermi-liquid ground state and exceeds the thermal conductivity by about 10%. This apparent violation of the WF law provides evidence for an unconventional type of QCP at which the fundamental concept of Landau quasiparticles breaks down. These results imply that Landau quasiparticles break up, and that the origin of this disintegration is inelastic scattering associated with electronic quantum critical fluctuations. Our finding brings new insights into understanding deviations from Fermi-liquid behaviour frequently observed in various classes of correlated materials.

cond-mat.str-el↗

arXiv2013

Lifshitz transitions and quasiparticle de-renormalization in YbRh$_2$Si$_2$

We study the effect of magnetic fields up to 15 T on the heavy fermion state of YbRh$_2$Si$_2$ via Hall effect and magnetoresistance measurements down to 50 mK. Our data show anomalies at three different characteristic fields. We compare our data to renormalized band structure calculations through which we identify Lifshitz transitions associated with the heavy fermion bands. The Hall measurements indicate that the de-renormalization of the quasiparticles, {\it i.e} the destruction of the local Kondo singlets, occurs smoothly while the Lifshitz transitions occur within rather confined regions of the magnetic field.

cond-mat.str-el↗

arXiv2013

Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnet Cs2CuCl4

We report magnetization and specific heat measurements in the 2D frustrated spin-1/2 Heisenberg antiferromagnet Cs2CuCl4 at temperatures down to 0.05 K and high magnetic fields up to 11.5 T applied along a, b and c-axes. The low-field susceptibility chi (T) M/B shows a broad maximum around 2.8 K characteristic of short-range antiferromagnetic correlations and the overall temperature dependence is well described by high temperature series expansion calculations for the partially frustrated triangular lattice with J=4.46 K and J'/J=1/3. At much lower temperatures (< 0.4 K) and in in-plane field (along b and c-axes) several new intermediate-field ordered phases are observed in-between the low-field incommensurate spiral and the high-field saturated ferromagnetic state. The ground state energy extracted from the magnetization curve shows strong zero-point quantum fluctuations in the ground state at low and intermediate fields.

cond-mat.str-el↗

arXiv2013

Field-induced suppression of the heavy-fermion state in YbRh_2Si_2

We report DC magnetization measurements on YbRh_2Si_2 at temperatures down to 0.04K, magnetic fields B<11.5T and under hydrostatic pressure P<1.3GPa. At ambient pressure a kink at B*=9.9T indicates a new type of field-induced transition from an itinerant to a localized 4f-state. This transition is different from the metamagnetic transition observed in other heavy fermion compounds, as here ferromagnetic rather than antiferromagnetic correlations dominate below B*. Hydrostatic pressure experiments reveal a clear correspondence of B* to the characteristic spin fluctuation temperature determined from specific heat.

cond-mat.str-el↗

arXiv2013

Interplay between Kondo suppression and Lifshitz transitions in YbRh$_2$Si$_2$ at high magnetic fields

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.

cond-mat.str-el↗

arXiv2013

Ferromagnetic order in Yb(Rh$_{0.73}$Co$_{0.27}$)$_{2}$Si$_{2}$

We present the discovery of ferromagnetism in single crystalline Yb(Rh$_{0.73}$Co$_{0.27}$)$_{2}$Si$_{2}$ below $T_{C} = 1.30\,$K with a spontaneous magnetic moment of $0.1\,μ_{B}$/Yb along the crystallographic c-axis. This is shown by the huge c-susceptibility peak, the hysteresis in magnetization as well as by the field-dependent susceptibility and specific heat data. This discovery motivates the reinvestigation of the magnetic order in Yb(Rh$_{1-x}$Co$_{x}$)$_{2}$Si$_{2}$ with $x < 0.27$ and in pure YbRh$_{2}$Si$_{2}$ under small pressures with field along the $c$-direction to look for low-lying ferromagnetic order.

cond-mat.str-el↗

arXiv2013

Far-infrared optical conductivity of CeCu2Si2

Journal ref.: J. Phys.: Condens. Matter 25, 065602 (2013): We investigated the optical reflectivity of the heavy-fermion metal CeCu2Si2 in the energy range 3 meV - 30 eV for temperatures between 4K - 300K. The results for the charge dynamics indicate a behavior that is expected for the formation of a coherent heavy quasiparticle state: Upon cooling the spectra of the optical conductivity indicate a narrowing of the coherent response. Below temperatures of 30 K a considerable suppression of conductivity evolves below a peak structure at 13 meV. We assign this gap-like feature to strong electron correlations due to the 4f-conduction electron hybridization.

cond-mat.str-el↗

arXiv2012

Multiband superconductivity in PrPt4Ge12 single crystals

We report measurements of the London penetration depth $Δλ(T)$ and the electronic specific heat $C_e(T)$ on high-quality single crystals of the filled-skutterudite superconductor PrPt$_4$Ge$_{12}$ ($T_c\simeq$8K). Both quantities show a weak temperature dependence at $T\ll T_c$, following $Δλ\sim T^n$ ($n\simeq 3.2$) and $C_e/T\sim T^{2.8}$. Such temperature dependences deviate from both conventional s-wave type and nodal superconductivity. A detailed analysis indicates that the superfluid density $ρ_s(T)$, derived from the penetration depth, as well as the electronic specific heat can be consistently described in terms of a two-gap model, providing strong evidence of multiband superconductivity for PrPt$_4$Ge$_{12}$.

cond-mat.supr-con↗

arXiv2012

Avoided ferromagnetic quantum critical point: Unusual short-range ordered state in CeFePO

Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac-susceptibility, specific heat and muon-spin relaxation ($μ$SR). Short-range static magnetism occurs below the freezing temperature Tg ~ 0.7 K, which prevents the system from accessing the putative ferromagnetic quantum critical point. In the $μ$SR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms.

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