arXiv subjects

G. Behr

Publications and source records attributed to G. Behr.

At least 19 recordsLinked to original sources

arXiv2017

Field-induced gapless electron pocket in the superconducting vortex phase of YNi2B2C as probed by magnetoacoustic quantum oscillations

By use of ultrasound studies we resolved magneto-acoustic quantum oscillation deep into the mixed state of the multiband nonmagnetic superconductor YNi2B2C. Below the upper critical field, only a very weak additional damping appears that can be well explained by the field inhomogeneity caused by the flux-line lattice in the mixed state. This is clear evidence for no or a vanishingly small gap for one of the bands, namely, the spheroidal alpha band. This contrasts de Haas--van Alphen data obtained by use of torque magnetometry for the same sample, with a rapidly vanishing oscillation signal in the mixed state. This points to a strongly distorted flux-line lattice in the latter case that, in general, can hamper a reliable extraction of gap parameters by use of such techniques.

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arXiv2016

Nematicity in LaFeAsO$_{1-x}$F$_x$

Orbital ordering has recently emerged as another important state in iron based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO$_{1-x}$F$_x$ with a focus on resistivity, Nernst effect, thermal expansion, and $^{75}$As NMR data. The transport properties at the underdoped level $x=0.05$ exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long-range nematic ordering, i.e. at about 160 K. Furthermore, the $^{75}$As-NMR spin-lattice relaxation rate $1/(T_1T)$ reveals a progressive slowing down of spin fluctuations. Yet, long-range magnetic order and also a detectable orthorhombic lattice distortion are absent. Thus, we conclude from the data that short-range orbital-nematic ordering or a slowly fluctuating form of it sets in near 160 K. Remarkably, all anomalies in the transport and also the indications of slow spin fluctuations disappear close to optimal doping $x=0.1$ which suggests that in LaFeAsO$_{1-x}$F$_x$ the nematic phase actually competes with superconductivity.

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arXiv2015

Spatial competition of the ground states in 1111 iron pnictides

Using nuclear quadrupole resonance, the phase diagram of 1111 $R$FeAsO$_{1-x}$F$_x$ ($R$$=$La, Ce, Sm) iron pnictides is constructed as a function of the local charge distribution in the paramagnetic state, which features low-doping-like (LD-like) and high-doping-like (HD-like) regions. Compounds based on magnetic rare earths (Ce, Sm) display a unified behavior, and comparison with La-based compounds reveals the detrimental role of static iron $3d$ magnetism on superconductivity, as well as a qualitatively different evolution of the latter at high doping. It is found that the LD-like regions fully account for the orthorhombicity of the system, and are thus the origin of any static iron magnetism. Orthorhombicity and static magnetism are not hindered by superconductivity but limited by dilution effects, in agreement with 2D (respectively 3D) nearest-neighbor square lattice site percolation when the rare earth is nonmagnetic (respectively magnetic). The LD-like regions are not intrinsically supportive of superconductivity, on the contrary of the HD-like regions, as evidenced by the well-defined Uemura relation between the superconducting transition temperature and the superfluid density when accounting for the proximity effect. This leads us to propose a complete description of the interplay of ground states in 1111 pnictides, where nanoscopic regions compete to establish the ground state through suppression of superconductivity by static magnetism, and extension of superconductivity by proximity effect.

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arXiv2013

Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study

Magnetic materials are usually divided into two classes: those with localised magnetic moments, and those with itinerant charge carriers. We present a comprehensive experimental (spectroscopic ellipsomerty) and theoretical study to demonstrate that these two types of magnetism do not only coexist but complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material the itinerant charge carriers interact with large localised magnetic moments of Tb(4f) states, forming complex magnetic lattices at low temperatures, which we associate with self-organisation of magnetic clusters. The formation of magnetic clusters results in low-energy optical spectral weight shifts, which correspond to opening of the pseudogap in the conduction band of the itinerant charge carriers and development of the low- and high-spin intersite electronic transitions. This phenomenon, driven by self-trapping of electrons by magnetic fluctuations, could be common in correlated metals, including besides Kondo-lattice metals, Fe-based and cuprate superconductors.

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arXiv2012

Pr magnetism and its interplay with the Fe spin density wave in PrFeAsO

We have studied the magnetism of the Pr3+ ions in PrFeAsO_1-xF_x (x = 0; 0.15) and its interaction with the Fe magnetic order (for x = 0). Specific heat data confirm the presence of a first excited crystal electric field (CEF) level around 3.5 meV in the undoped compound PrFeAsO. This finding is in agreement with recent neutron scattering experiments. The doped compound is found to have a much lower first CEF splitting of about 2.0 meV. The Pr ordering in PrFeAsO gives rise to large anomalies in the specific heat and the thermal expansion coefficient. In addition, a field-induced transition is found at low temperatures that is most pronounced for the magnetostriction coefficient. This transition, which is absent in the doped compound, is attributed to a reversal of the Fe spin canting as the antiferromagnetic Pr order is destroyed by the external magnetic field.

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arXiv2012

Hole-doping in BaFe$_2$As$_2$: The case of Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals

Single crystals of Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ with $x$ = 0, 0.25, 0.35, 0.4 were grown using a self-flux high temperature solution growth technique. The superconducting and normal state properties were studied by temperature dependent magnetic susceptibility, electrical resistivity and specific heat revealing that the magnetic and structural transition is rapidly suppressed upon Na-substitution at the Ba-site in BaFe$_2$As$_2$, giving rise to superconductivity. A superconducting transition as high as 34 K is reached for a Na-content of $x$=0.4. The positive Hall coefficient confirms that the substitution of Ba by Na results in hole-doping similarly to the substitution of Ba by K. Angle resolved photoemission spectroscopy was performed on all Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ crystals. The Fermi surface of hole-doped Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ is to high extent the same as the Fermi surface found for the K-doped sister compounds, suggesting a similar impact of the substitution of Ba by either K or Na on the electronic band dispersion at the Fermi level

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arXiv2011

Bond disorder and breakdown of ballistic heat transport in the spin-1/2 antiferromagnetic Heisenberg chain as seen in Ca-doped SrCuO2

We study the impact of a weak bond disorder on the spinon heat transport in the S=1/2 antiferromagnetic (AFM) Heisenberg chain material Sr_{1-x}Ca_xCuO_2. We observe a drastic suppression in the magnetic heat conductivity kappa_mag even at tiny disorder levels (i.e., Ca-doping levels), in stark contrast to previous findings for kappa_mag of S=1/2 two-dimensional square lattice and two-leg spin-ladder systems, where a similar bond disorder has no effect on kappa_mag. Hence, our results underpin the exceptional role of integrability of the S=1/2 AFM Heisenberg chain model and suggest that the bond disorder effectively destroys the ballistic nature of its heat transport. We further show that the suppression of kappa_mag is captured by an effective spinon-impurity scattering length, which exhibits the same doping dependence as the long-distance exponential decay length of the spin-spin correlation as determined by density-matrix renormalization group calculations.

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arXiv2011

Peculiarities of the superconducting gaps and the electron-boson interaction in TmNi2B2C as seen by point-contact spectroscopy

Point-contact (PC) investigations on the title compound in the normal and superconducting (SC) state (Tc=10,6 K) are presented. The temperature dependence of the SC gap of TmNi2B2C determined from Andreev-reflection (AR) spectra using the standard single-gap approximation (SGA) deviates from the BCS behavior in displaying a maximum at about Tc/2. A refined analysis within the two-gap approximation provides evidence for the presence of a second gap twice as large as the main gap (the first one), while the latter is close to that within the SGA. This way, TmNi2B2C expands the number of nickel borocarbide superconductors which exhibit a clear multiband character. Additionally, for the first time "reentrant" features were found in the AR spectra for some PCs measured in a magnetic field. The PC spectroscopy of the electron-boson interaction in TmNi2B2C in the normal state reveals a pronounced phonon maximum at 9.5meV and a more smeared one around 15 meV, while at higher energies the PC spectra are almost featureless. Additionally, the most intense peak slightly above 3meV observed in the PC spectra of TmNi2B2C is presumably caused by crystalline-electric-field (CEF) excitations. The peak near 1meV detected for some PC spectra is connected with a modification of the CEF probably due to boron or carbon vacancies, allowing to probe the local stoichiometry by PC spectroscopy.

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arXiv2011

High-field electron spin resonance spectroscopy study of GdO$_{1-x}$F$_{x}$FeAs superconductors

We report a detailed investigation of GdO$_{1-x}$F$_{x}$FeAs (x = 0, 0.07 and 0.14) samples by means of high-field/high-frequency electron spin resonance (HF-ESR) together with measurements of thermodynamic and transport properties. The parent GdOFeAs compound exhibits Fe long-range magnetic order below 128 K, whereas both doped samples do not show such order and are superconducting with T$_c$ = 20 K (x = 0.07) and T$_c$ = 45 K (x = 0.14). The Gd$^{3+}$ HF-ESR reveals an appreciable exchange coupling between Gd and Fe moments, through which the static magnetic order is clearly seen in the parent compound. Owing to this coupling, HF-ESR can probe sensitively the evolution of the magnetism in the FeAs planes upon F doping. It is found that in both superconducting samples, where the Fe long-range order is absent, there are short-range, static on the ESR time scale magnetic correlations between Fe spins. Their occurrence on a large doping scale may be indicative of the ground states' coexistence.

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arXiv2011

Time Resolved Stroboscopic Neutron Scattering of Vortex Lattice Dynamics in Superconducting Niobium

Superconducting vortex lattices, glasses and liquids attract great interest as model systems of crystallization and as a source of microscopic information of the nature of superconductivity. We report for the first time direct microscopic measurements of the vortex lattice tilt modulus c44 in ultra-pure niobium using time-resolved small angle neutron scattering. Besides a general trend to faster vortex lattice dynamics for increasing temperatures we observe a dramatic changeover of the relaxation process associated with the non-trivial vortex lattice morphology in the intermediate mixed state. This changeover is attributed to a Landau-branching of the Shubnikov domains at the surface of the sample. Our study represents a showcase for how to access directly vortex lattice melting and the formation of vortex matter states for other systems.

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arXiv2010

Phase diagram of iron-arsenide superconductors Ca(Fe1-xCox)2As2 (0 <= x <= 0.2)

Platelet-like single crystals of the Ca(Fe1-xCox)2As2 series having lateral dimensions up to 15 mm and thickness up to 0.5 mm were obtained from the high temperature solution growth technique using Sn flux. Upon Co doping, the c-axis of the tetragonal unit cell decreases, while the a-axis shows a less significant variation. Pristine CaFe2As2 shows a combined spin-density-wave and structural transition near T = 166 K which gradually shifts to lower temperatures and splits with increasing Co-doping. Both transitions terminate abruptly at a critical Co-concentration of xc = 0.075. For x \geq 0.05, superconductivity appears at low temperatures with a maximum transition temperature TC of around 20 K. The superconducting volume fraction increases with Co concentration up to x = 0.09 followed by a gradual decrease with further increase of the doping level. The electronic phase diagram of Ca(Fe1-xCox)2As2 (0 \leq x \leq 0.2) series is constructed from the magnetization and electric resistivity data. We show that the low-temperature superconducting properties of Co-doped CaFe2As2 differ considerably from those of BaFe2As2 reported previously. These differences seem to be related to the extreme pressure sensitivity of CaFe2As2 relative to its Ba counterpart.

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arXiv2010

Damping of dHvA oscillations and vortex-lattice disorder in the peak-effect region of strong type-II superconductors

The phenomenon of magnetic quantum oscillations in the superconducting state poses several questions that still defy satisfactory answers. A key controversial issue concerns the additional damping observed in the vortex state. Here, we show results of μSR, dHvA, and SQUID magnetization measurements on borocarbide superconductors, indicating that a sharp drop observed in the dHvA amplitude just below H_{c2} is correlated with enhanced disorder of the vortex lattice in the peak-effect region, which significantly enhances quasiparticle scattering by the pair potential.

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arXiv2010

Nanoscale electronic order in iron pnictides

The charge distribution in RFeAsO$_{1-x}$F$_x$ (R=La, Sm) iron pnictides is probed using As nuclear quadrupole resonance. Whereas undoped and optimally-doped or overdoped compounds feature a single charge environment, two charge environments are detected in the underdoped region. Spin-lattice relaxation measurements show their coexistence at the nanoscale. Together with the quantitative variations of the spectra with doping, they point to a local electronic order in the iron layers, where low- and high-doping-like regions would coexist. Implications for the interplay of static magnetism and superconductivity are discussed.

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arXiv2010

Observation of phonons with resonant inelastic x-ray scattering

Phonons, the quantum mechanical representation of lattice vibrations, and their coupling to the electronic degrees of freedom are important for understanding thermal and electric properties of materials. For the first time, phonons have been measured using resonant inelastic x-ray scattering (RIXS) across the Cu K-edge in cupric oxide (CuO). Analyzing these spectra using an ultra-short core-hole lifetime approximation and exact diagonalization techniques, we can explain the essential inelastic features. The relative spectral intensities are related to the electron-phonon coupling strengths.

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arXiv2010

Scaling of normal-state transport properties of 1111 iron-pnictide superconductors

We show that the zero field normal-state resistivity above Tc for various levels of electron doping-both for underdoped, optimally, and overdoped LaO1-xFxFeAs, i.e. (x=0.05-0.075 and 0.1-0.2) and underdoped SmO1-xFxFeAs (x=0.06- 0.1) members of the 1111-iron-pnictide superconductor family-can be scaled in a broad temperature range from 20 to 300 K onto a single universal curve. The scaling was performed using the energy scale Delta, the resistivity rho(Delta) and the residual resistivity rho0 as scaling parameters as well as applying a recently proposed model-independent scaling method (H. G. Luo, Y. H. Su, and T. Xiang, Phys. Rev. B 77, 014529 (2008)). The scaling parameters have been calculated and the compositional variation of Delta has been determined. The existence of a universal metallic rho(T) curve is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in LaO1-xFxFeAs (x= 0.05 to 0.075 and 0.1 to 0.2) and SmO1-xFxFeAs (x = 0.06 to 0.1) .

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arXiv2010

Critical current and vortex dynamics in single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$

We investigate the critical current density and vortex dynamics in single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ ($x$ = 0.051, 0.056, 0.065, and 0.073). The samples exhibit different critical temperatures and superconducting phase fractions. We show that in contrast to their Ba-based counterpart, the crystals do not exhibit a second peak in the field dependence of magnetization. The calculated composition-dependent critical current density ($j_{\rm c}$) increases initially with Co doping, maximizing at $x$ = 0.065, and then decreases. This variation in $j_{\rm c}$ follows the superconducting phase fractions in this series. The calculated $j_{\rm c}$ shows strong temperature dependence, decreasing rapidly upon heating. Magnetic relaxation measurements imply a nonlogarithmic dependence on time. We find that the relaxation rate is large, reflecting weak characteristic pinning energy. The analysis of temperature- and field-dependent magnetic relaxation data suggests that vortex dynamics in these compounds is consistent with plastic creeping rather than the collective creep model, unlike other 122 pnictide superconductors. This difference may cause the absence of the second peak in the field dependent magnetization of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$.

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arXiv2010

Unusual disorder effects in superconducting LaFeAs_(1-d)O_0.9F_0.1 as revealed by NMR spectroscopy

We report 75As NMR measurements of the spin-lattice relaxation in the superconducting state of LaFeAsO_(0.9)F_(0.1) and As-deficient LaFeAs_(1-d)O_0.9F_0.1. The temperature behavior of 1/T_1 below T_c changes drastically from a T^3-dependence for LaFeAsO_(0.9)F_(0.1) to a T^5-dependence for the As-deficient sample. These results, together with the previously reported unexpected increase of T_c and the slope of the upper critical field near T_c for the As-deficient sample, are discussed in terms of non-universal SC gaps in Fe-pnictides and the effect of As deficiency as an exotic case where nonmagnetic 'smart' impurities even stabilize an s+- wave superconductor or within a scenario of a disorder-driven change to s++ superconductivity.

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arXiv2010

Time of life as it is in LiFeAs

The time of life of fermionic quasiparticles, the distribution of which in the momentum-energy space can be measured by angle resolved photoemission (ARPES), is the first quantity to look for fingerprints of interaction responsible for the superconducting pairing. Such an approach has been recently used for superconducting cuprates, but its direct application to pnictides was not possible due to essential three-dimensionality of the electronic band structure and magnetic ordering. Here, we report the investigation of the quasiparticle lifetime in LiFeAs, a non-magnetic stoichiometric superconductor with a well separated two-dimensional band. We have found two energy scales: the lower one contains clear fingerprints of optical phonon modes while the higher scale indicates a presence of strong electron-electron interaction. The result suggests that LiFeAs is a phonon mediated superconductor with strongly enhanced electronic density of states at the Fermi level.

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