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D. Mandrus

Publications and source records attributed to D. Mandrus.

At least 73 records · Page 4Linked to original sources
arXiv2011

Phonon splitting and anomalous enhancement of infrared-active modes in BaFe$_2$As$_2$

We present a comprehensive infrared spectroscopic study of lattice dynamics in the pnictide parent compound BaFe$_2$As$_2$. In the tetragonal structural phase, we observe the two degenerate symmetry-allowed in-plane infrared active phonon modes. Following the structural transition from the tetragonal to orthorhombic phase, we observe splitting into four non-degenerate phonon modes and a significant phonon strength enhancement. These detailed data allow us to provide a physical explanation for the anomalous phonon strength enhancement as the result of anisotropic conductivity due to Hund's coupling.

cond-mat.supr-con↗
arXiv2011

Electronic correlations and unconventional spectral weight transfer in BaFe$_{2-x}$Co$_{x}$As$_{2}$

We report an infrared optical study of the pnictide high-temperature superconductor BaFe$_{1.84}$Co$_{0.16}$As$_{2}$ and its parent compound BaFe$_{2}$As$_{2}$. We demonstrate that electronic correlations are moderately strong and do not change across the spin-density wave transition or with doping. By examining the energy scale and direction of spectral weight transfer, we argue that Hund's coupling \emph{J} is the primary mechanism that gives rise to correlations.

cond-mat.str-el↗
arXiv2011

Spatial Inhomogeneity in RFeAsO1-xFx (R =Pr, Nd) Determined from Rare Earth Crystal Field Excitations

We report inelastic neutron scattering measurements of crystal field transitions in PrFeAsO, PrFeAsO0.87F0.13, and NdFeAsO0.85F0.15. Doping with fluorine produces additional crystal field excitations, providing evidence that there are two distinct charge environments around the rare earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr3+ and Nd3+ ions have non-magnetic and magnetic ground states, respectively, indicating that the enhancement of Tc compared to LaFeAsO1-xFx is not due to rare earth magnetism.

cond-mat.supr-con↗
arXiv2010

Magnetism and Disorder Effects on muSR Measurements of the Magnetic Penetration Depth in Iron-Based Superconductors

It is shown that attempts to accurately deduce the magnetic penetration depth of overdoped BaFe_{1.82}Co_{0.18}As2 single crystals by transverse-field muon spin rotation (TF-muSR) are thwarted by field-induced magnetic order and strong vortex-lattice disorder. We explain how substantial deviations from the magnetic field distribution of a nearly perfect vortex lattice by one or both of these factors is also significant for other iron-based superconductors, and this introduces considerable uncertainty in the values of the magnetic penetration depth obtained by TF-muSR.

cond-mat.supr-con↗
arXiv2010

Effect of annealing on the specific heat of optimally doped Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$

We report the temperature dependence of the low-temperature specific heat down to 400 mK of the electron-doped Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$ superconductors. We have measured two samples extracted from the same batch: first sample has been measured just after preparation with no additional heat treatment. The sample shows $T_{c}$=20 K, residual specific heat $γ_{0}$=3.6 mJ/mol K$^{2}$ and a Schottky-like contribution at low temperatures. A second sample has been annealed at 800 $^{o}C$ for two weeks and shows $T_{c}$ = 25 K and $γ_{0}$=1.4 mJ/mol~K$^{2}$. By subtracting the lattice specific heat, from pure BaFe$_{2}$As$_{2}$, the temperature dependence of the electronic specific heat has been obtained and studied. For both samples the temperature dependence of $C_{el}(T)$ clearly indicate the presence of low-energy excitations in the system. Their specific heat data cannot be described by single clean s- or d-wave models and the data requires an anisotropic gap scenario which may or may not have nodes

cond-mat.supr-con↗
arXiv2010

Electronic Structure of CeFeAsO1-xFx (x=0, 0.11/x=0.12) compounds

We report an extensive study on the intrinsic bulk electronic structure of the high-temperature superconductor CeFeAsO0.89F0.11 and its parent compound CeFeAsO by soft and hard x-ray photoemission, x-ray absorption and soft-x-ray emission spectroscopies. The complementary surface/bulk probing depth, and the elemental and chemical sensitivity of these techniques allows resolving the intrinsic electronic structure of each element and correlating it with the local structure, which has been probed by extended-x-ray absorption fine structure spectroscopy. The measurements indicate a predominant 4f1 (i.e. Ce3+) initial state configuration for Cerium and an effective valence-band-to-4f charge-transfer screening of the core hole. The spectra also reveal the presence of a small Ce f0 initial state configuration, which we assign to the occurrence of an intermediate valence state. The data reveal a reasonably good agreement with the partial density of states as obtained in standard density functional calculations over a large energy range. Implications for the electronic structure of these materials are discussed.

cond-mat.supr-con↗
arXiv2010

A doping dependent specific heat study of the superconducting gap in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

We report a doping, magnetic field and low-temperature dependent study of the specific heat of the iron-arsenide Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ at under (x=0.045), optimal (x=0.08) and overdoped (x=0.103 and 0.105) regimes. By subtracting the lattice specific heat the temperature and magnetic field dependence of the electronic specific heat has been studied. The temperature and field dependencies of the superconducting part of $C_{p}$ exhibit similar behavior for all doping concentrations. The temperature variation of the electronic specific heat as well as its field dependence cannot be described by a single isotropic s-wave gap, pointing to a complex gap structure in the system. The lack of doping dependence indicates that the gap structure does not change significantly as a function of doping. We also observe a significant residual linear term of unknown origin in the specific heat of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ which suggests that inhomogeneity may be an important factor in Co-doped BaFe$_{2}$As$_{2}$

cond-mat.supr-con↗
arXiv2010

Orbital Symmetry of Ba(Fe1-xCox)2As2 Superconductors Probed with X-ray Absorption Spectroscopy

The orbital symmetries of electron doped iron-arsenide superconductors Ba(Fe1-xCox)2As2 have been measured with x-ray absorption spectroscopy. The data reveal signatures of Fe d electron itinerancy, weak electronic correlations, and a high degree of Fe-As hybridization related to the bonding topology of the Fe dxz+yz states, which are found to contribute substantially at the Fermi level. The energies and detailed orbital character of Fe and As derived unoccupied s and d states are found to be in remarkably good agreement with the predictions of standard density functional theory.

cond-mat.supr-con↗
arXiv2010

Probing Local Variations of Superconductivity on the Surface of Ba(Fe1-xCox)2As2 Single Crystals

The spatially resolved electrical transport properties have been studied on the surface of optimally-doped superconducting Ba(Fe1-xCox)2As2 single crystal by using a four-probe scanning tunneling microscopy. While some non-uniform contrast appears near the edge of the cleaved crystal, the scanning electron microscopy (SEM) reveals mostly uniform contrast. For the regions that showed uniform SEM contrast, a sharp superconducting transition at TC = 22.1 K has been observed with a transition width (delta)Tc = 0.2 K. In the non-uniform contrast region, TC is found to vary between 19.6 and 22.2 K with (delta)Tc from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co concentration remains 7.72% in the uniform region, but changes between 7.38% and 7.62% in the non-uniform region. Thus the variations of superconductivity are associated with local compositional change.

cond-mat.supr-con↗
arXiv2010

Gap structure in the electron-doped Iron-Arsenide Superconductor Ba(Fe0.92Co0.08)2As2: low-temperature specific heat study

We report the field and temperature dependence of the low-temperature specific heat down to 400 mK and in magnetic fields up to 9 T of the electron-doped Ba(Fe0.92Co0.08)2As2 superconductor. Using the phonon specific heat obtained from pure BaFe2As2 we find the normal state Sommerfeld coefficient to be 18 mJ/mol.K^2 and a condensation energy of 1.27 J/mol. The temperature dependence of the electronic specific heat clearly indicate the presence of the low-energy excitations in the system. The magnetic field variation of field-induced specific heat cannot be described by single clean s- or d-wave models. Rather, the data require an anisotropic gap scenario which may or may not have nodes. We discuss the implications of these results.

cond-mat.supr-con↗
arXiv2010

Contrasting Spin Dynamics Between Underdoped and Overdoped Ba(Fe[1-x]Co[x])2As2

We report the first NMR investigation of spin dynamics in the overdoped non-superconducting regime of Ba(Fe[1-x]Co[x])2As2 up to x =0.26. We demonstrate that the absence of inter-band transitions with large momentum transfer Q(AF) ~ (pi, 0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x > 0.15. Our experimental results provide direct evidence for a correlation between Tc and the strength of Q(AF) antiferromagnetic spin fluctuations.

cond-mat.supr-con↗
arXiv2009

Unusual Relationship between Magnetism and Superconductivity in FeTe$_{0.5}$Se$_{0.5}$

We use neutron scattering, to study magnetic excitations in crystals near the ideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of excitations are found, a resonance at (0.5, 0.5, 0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials.

cond-mat.supr-con↗
arXiv2009

Temperature and Pressure Dependence of the Fe-specific Phonon Density of States in Ba(Fe(1-x)Co(x))2As2

The {57}Fe-specific phonon density of states of Ba(Fe(1-x)Co(x))2As2 single crystals (x=0.0, 0.08) was measured at cryogenic temperatures and at high pressures with nuclear-resonant inelastic x-ray scattering. Measurements were conducted for two different orientations of the single crystals, yielding the orientation-projected {57}Fe-phonon density of states (DOS) for phonon polarizations in-plane and out-of-plane with respect to the basal plane of the crystal structure. In the tetragonal phase at 300 K, a clear stiffening was observed upon doping with Co. Increasing pressure to 4 GPa caused a marked increase of phonon frequencies, with the doped material still stiffer than the parent compound. Upon cooling, both the doped and undoped samples showed a stiffening, and the parent compound exhibited a discontinuity across the magnetic and structural phase transition. These findings are generally compatible with the changes in volume of the system upon doping, increasing pressure, or increasing temperature, but an extra softening of high-energy modes occurs with increasing temperature. First-principles computations of the phonon DOS were performed and showed an overall agreement with the experimental results, but underestimate the Grueneisen parameter. This discrepancy is explained in terms of a magnetic Grueneisen parameter, causing an extra phonon stiffening as magnetism is suppressed under pressure.

cond-mat.supr-con↗
arXiv2009

Point-contact spectroscopic studies on normal and superconducting AFe_2As_2-type iron-pnictide single crystals

Point-contact Andreev reflection spectroscopy (PCARS) is applied to investigate the gap structure in iron pnictide single crystal superconductors of the AFe_2As_2 (A=Ba, Sr) family ("Fe-122"). The observed point-contact junction conductance curves, G(V), can be divided into two categories: one where Andreev reflection is present for both (Ba_{0.6}K_{0.4})Fe_2As_2 and Ba(Fe_{0.9}Co_{0.1})_2As_2, and the other with a V^{2/3} background conductance universally observed extending even up to 100 meV for Sr_{0.6}Na_{0.4}Fe_2As_2 and Sr(Fe_{0.9}Co_{0.1})_2As_2. The latter is also observed in point-contact junctions on the nonsuperconducting parent compound BaFe_2As_2. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors in the superconducting samples. For Ba_{0.6}K_{0.4}Fe_2As_2, double peaks due to Andreev reflection with strongly-sloping background are frequently observed for point-contacts on freshly-cleaved c-axis surfaces. If normalized by a background baseline and analyzed by the Blonder-Tinkham-Klapwijk model, the data show a gap size ~3.0-4.0 meV with 2Δ_0/k_BT_c ~ 2.0-2.6, consistent with the smaller gap size reported in the LnFeAsO family ("Fe-1111"). For the Ba(Fe_{0.9}Co_{0.1})_2As_2, G(V) curves typically display a zero-bias conductance peak.

cond-mat.supr-con↗
arXiv2009

Infrared anomalous Hall effect in SrRuO$_3$: Evidence for crossover to intrinsic behavior

The origin of the Hall effect in many itinerant ferromagnets is still not resolved, with an anomalous contribution from the sample magnetization that can exhibit extrinsic or intrinsic behavior. We report the first mid-infared (MIR) measurements of the complex Hall ($θ_H$), Faraday ($θ_F$), and Kerr ($θ_K$) angles, as well as the Hall conductivity ($σ_{xy}$) in a SrRuO$_3$ film in the 115-1400 meV energy range. The magnetic field, temperature, and frequency dependence of the Hall effect is explored. The MIR magneto-optical response shows very strong frequency dependence, including sign changes. Below 200 meV, the MIR $θ_H (T)$ changes sign between 120 and 150 K, as is observed in dc Hall measurements. Above 200 meV, the temperature dependence of $θ_H$ is similar to that of the dc magnetization and the measurements are in good agreement with predictions from a band calculation for the intrinsic anomalous Hall effect (AHE). The temperature and frequency dependence of the measured Hall effect suggests that whereas the behavior above 200 meV is consistent with an intrinsic AHE, the extrinsic AHE plays an important role in the lower energy response.

cond-mat.str-el↗
arXiv2009

A Model of the Normal State Susceptibility and Transport Properties of Ba (Fe1-xCox)2As2: An Explanation of the Increase of Magnetic Susceptibility with Temperature

A simple two-band model is used to describe the magnitude and temperature dependence of the magnetic susceptibility, Hall coefficient and Seebeck data from undoped and Co doped BaFe2As2. Overlapping rigid parabolic electron and hole bands are considered as a model of the electronic structure of the FeAs-based semimetals. The model has only three parameters: the electron and hole effective masses and the position of the valence band maximum with respect to the conduction band minimum. The model is able to reproduce in a semiquantitative fashion the magnitude and temperature dependence of many of the normal state magnetic and transport data from the FeAs-type materials, including the ubiquitous increase in the magnetic susceptibility with increasing temperature.

cond-mat.supr-con↗
arXiv2009

Anisotropic thermal expansion of Fe1.06Te and FeTe0.5Se0.5 single crystals

Heat capacity and anisotropic thermal expansion was measured for Fe1.06Te and FeTe0.5Se0.5 single crystals. Previously reported phase transitions are clearly seen in both measurements. In both cases the thermal expansion is anisotropic. The uniaxial pressure derivatives of the superconducting transition temperature in FeTe0.5Se0.5 inferred from the Ehrenfest relation have opposite signs for in-plane and c-axis pressures. Whereas the Gruneisen parameters for both materials are similar and only weakly temperature-dependent above ~ 80 K, at low temperatures (in the magnetically ordered phase) the magnetic contribution to the Gruneisen parameter in Fe1.06Te is significantly larger than electron and phonon contributions combined.

cond-mat.supr-con↗