arXiv subjects

A. S. Sefat

Publications and source records attributed to A. S. Sefat.

At least 55 records · Page 3Linked to original sources

arXiv2014

Critical Behavior of the SDW Transition in Underdoped Ba(Fe1-xCox)2As2 (x <=0.05): 75As NMR Investigation

We investigate the nature of the SDW (Spin Density Wave) transition in the underdoped regime of an iron-based high Tc superconductor Ba(Fe1-xCox)2As2 by 75As NMR, with primary focus on a composition with x = 0.02 (T_SDW = 99 K).We demonstrate that critical slowing down toward the three dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition, Ts = 105 K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between Ts and T_SDW, the dynamical structure factor of electron spins S(q,Wn) measured with the longitudinal NMR relaxation rate 1/T1 exhibits a divergent behavior obeying a power law, 1/T1~S(q, Wn)~(T/T_SDW-1)^a with the critical exponent a ~ 0.33.

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arXiv2014

The evolution of antiferromagnetic susceptibility to uniaxial pressure in Ba(Fe{1-x}Co{x})2As2

Neutron diffraction measurements are presented measuring the responses of both magnetic and structural order parameters of parent and lightly Co-doped Ba(Fe{1-x}Co{x})2As2 under the application of uniaxial pressure. We find that the uniaxial pressure induces a thermal shift in the onset of antiferromagnetic order that grows as a percentage of T_N as Co-doping is increased and the superconducting phase is approached. Additionally, as uniaxial pressure is increased within parent and lightly-doped Ba(Fe{1-x}Co{x})2As2 on the first order side of the tricritical point, we observe a decoupling between the onsets of the orthorhombic structural distortion and antiferromagnetism. Our findings place needed constraints on models exploring the nematic susceptibility of the bilayer pnictides in the tetragonal, paramagnetic regime.

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arXiv2014

On infrared pseudogap in cuprate and pnictide high-temperature superconductors

We investigate infrared manifestations of the pseudogap in the prototypical cuprate and pnictide superconductors: YBa2Cu3Oy and BaFe2As2 (Ba122) systems. We find remarkable similarities between the spectroscopic features attributable to the pseudogap in these two classes of superconductors. The hallmarks of the pseudogap state in both systems include a weak absorption feature at about 500 cm-1 followed by a featureless continuum between 500 and 1500 cm-1 in the conductivity data and a significant suppression in the scattering rate below 700 - 900 cm-1. The latter result allows us to identify the energy scale associated with the pseudogap $Δ_{PG}$. We find that in the Ba122-based materials the superconductivity-induced changes of the infrared spectra occur in the frequency region below 100 - 200 cm-1, which is much lower than the energy scale of the pseudogap. We performed theoretical analysis of the scattering rate data of the two compounds using the same model which accounts for the effects of the pseudogap and electron-boson coupling. We find that the scattering rate suppression in Ba122-based compounds below $Δ_{PG}$ is solely due to the pseudogap formation whereas the impact of the electron-boson coupling effects is limited to lower frequencies. The magnetic resonance modes used as inputs in our modeling are found to evolve with the development of the pseudogap, suggesting an intimate correlation between the pseudogap and magnetism.

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arXiv2014

Fermi-Surface Reconstruction and Complex Phase Equilibria in CaFe$_{2}$As$_{2}$

Fermi-surface topology governs the relationship between magnetism and superconductivity in iron-based materials. Using low-temperature transport, angle-resolved photoemission, and x-ray diffraction we show unambiguous evidence of large Fermi surface reconstruction in CaFe$_{2}$As$_{2}$ at magnetic spin-density-wave and nonmagnetic collapsed-tetragonal ($cT$) transitions. For the $cT$ transition, the change in the Fermi surface topology has a different character with no contribution from the hole part of the Fermi surface. In addition, the results suggest that the pressure effect in CaFe$_{2}$As$_{2}$ is mainly leading to a rigid-band-like change of the valence electronic structure. We discuss these results and their implications for magnetism and superconductivity in this material.

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arXiv2014

Crossover from spin-waves to diffusive spin excitations in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from x=0.04-0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials.

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arXiv2013

Angle-resolved photoemission spectroscopy observation of anomalous electronic states in EuFe$_2$As$_{2-x}$P$_x$

We used angle-resolved photoemission spectroscopy to investigate the electronic structure of EuFe$_2$As$_2$, EuFe$_2$As$_{1.4}$P$_{0.6}$ and EuFe$_2$P$_2$. We observed doubled core level peaks associated to the pnictide atoms, which are related to a surface state. Nevertheless, strong electronic dispersion along the $c$ axis, especially pronounced in EuFe$_2$P$_2$, is observed for at less one band, thus indicated that the Fe states, albeit probably affected at the surface, do not form pure two-dimensional surface states. However, this latter material shows reduced spectral weight near the Fermi level as compared to EuFe$_2$As$_2$ and EuFe$_2$As$_{1.4}$P$_{0.6}$. An anomalous jump is also found in the electronic states associated with the Eu$^{2+}$ $f$ states in EuFe$_2$P$_2$.

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arXiv2013

Magnetic structure and spin excitations in BaMn2Bi2

We present a single crystal neutron scattering study of BaMn2Bi2, a recently synthesized material with the same ThCr2Si2-type structure found in several Fe-based unconventional superconducting materials. We show long range magnetic order, in the form of a G-type antiferromagnetic structure, to exist up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering we observe a spin-gap of 16meV, with spin-waves extending up to 55 meV. We find these magnetic excitations to be well fit to a J1-J2-Jc Heisenberg model and present values for the exchange interactions. The spin wave spectrum appears to be unchanged by the 100 K structural phase transition.

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arXiv2013

Absence of structural transition in TM0.5IrTe2 (TM=Mn, Fe, Co, Ni)

TM-doped IrTe2(TM=Mn, Fe, Co, Ni) compounds were synthesized by solid state reaction. Single crystal x-ray diffraction experiments indicate that part of the doped TM ions (TM=Fe, Co, and Ni) substitute for Ir, and the rest intercalate into the octahedral interstitial sites located in between IrTe2 layers. Due to the lattice mismatch between MnTe2 and IrTe2, Mn has limited solubility in IrTe2 lattice. The trigonal structure is stable in the whole temperature range 1.80<T<300K for all doped compositions. No long range magnetic order or superconductivity was observed in any doped compositions above 1.80K. A spin glass behavior below 10K was observed in Fe-doped IrTe2 from the temperature dependence of magnetization, electrical resistivity, and specific heat. The low temperature specific heat data suggest the electron density of states is enhanced in Fe- and Co-doped compositions but reduced in Ni-doped IrTe2. With the 3d transition metal doping the trigonal a-lattice parameters increases but the c-lattice parameter decreases. Detailed analysis of the single crystal x-ray diffraction data shows that interlayer Te-Te distance increases despite a reduced c-lattice. The importance of the Te-Te, Te-Ir, and Ir-Ir bonding is discussed.

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arXiv2013

Doping dependence of the spin excitations in Fe-based superconductors Fe1+yTe1-xSex

The Fe1+yTe1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials we present systematic inelastic neutron scattering measurements of the low energy spin excitations for x=0.27, 0.36, 0.40, 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2 1/2 0) wave vector with doping. Concentrations (x=0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2 1/2 0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2 1/2 0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2 1/2 0) wave vector and the role that interstitial Fe may play.

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arXiv2013

Intrinsic superconducting properties and vortex dynamics in heavily overdoped Ba(Fe$_{0.86}$Co$_{0.14}$)$_2$As$_2$ single crystal

In this work we report the influence of intrinsic superconducting parameters on the vortex dynamics in an overdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (x=0.14) single crystal. We find a superconducting critical temperature of 13.5 K, magnetic penetration depth $λ_{ab}$(0) = 660 $\pm$ 50 nm, coherence length $ξ_{ab}$(0) = 5 nm, and the upper critical field anisotropy $γ_{T\rightarrow Tc}$ $\approx$ 3.7. In fact, the Ginzburg-Landau model may explain the angular dependent $H_{c2}$ for this anisotropic three-dimensional superconductor. The vortex phase diagram, in comparison with the optimally doped compound, presents a narrow collective creep regime. In addition, we found no sign of correlated pinning along the c axis. Our results show that vortex core to defect size ratio and $λ$ play an important role in the resulting vortex dynamics in materials with similar intrinsic thermal fluctuations.

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arXiv2012

NMR Search for the Spin Nematic State in LaFeAsO Single Crystal

We report a 75-As single crystal NMR investigation of LaFeAsO, the parent phase of a pnictide high Tc superconductor. We demonstrate that spin dynamics develop a strong two-fold anisotropy within each orthorhombic domain below the tetragonal-orthorhombic structural phase transition at T[TO]~156 K. This intermediate state with a dynamical breaking of the rotational symmetry freezes progressively into a spin density wave (SDW) below T[SDW]~142 K. Our findings are consistent with the presence of a spin nematic state below T[TO] with an incipient magnetic order.

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arXiv2012

Electronic band structure of BaCo$_{2}$As$_2$: a fully-doped ferropnictide with reduced electronic correlations

We report an angle-resolved photoemission spectroscopy investigation of the Fermi surface and electronic band structure of BaCo$_{2}$As$_2$. Although its quasi-nesting-free Fermi surface differs drastically from that of its Fe-pnictide cousins, we show that the BaCo$_{2}$As$_2$ system can be used as an approximation to the bare unoccupied band structure of the related BaFe$_{2-x}$Co$_x$As$_2$ and Ba$_{1-x}$K$_x$Fe$_2$As$_2$ compounds. However, our experimental results, in agreement with dynamical mean field theory calculations, indicate that electronic correlations are much less important in BaCo$_{2}$As$_2$ than in the ferropnictides. Our findings suggest that this effect is due to the increased filling of the electronic 3$d$ shell in the presence of significant Hund's exchange coupling.

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arXiv2012

Infrared Measurement of the Pseudogap in P-Doped and Co-Doped BaFe2As2 High-Temperature Superconductors

We report on infrared studies of charge dynamics in a prototypical pnictide system: the BaFe2As2 family. Our experiments have identified hallmarks of the pseudogap state in the BaFe2As2 system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.

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arXiv2012

Specific Heat To Hc2: Evidence for Nodes or Deep Minima in the Superconducting Gap of Under- and Overdoped Ba(Fe1-xCox)2As2

Low temperature specific heat, C, in magnetic fields up to Hc2 is reported for underdoped Ba(Fe0.955Co0.045)2As2 (Tc=8 K) and for three overdoped samples Ba(Fe1-xCox)2As2 (x=0.103, 0.13, and 0.15, Tc=17.2, 16.5, and 11.7 K respectively). Previous measurements of thermal conductivity (as a function of temperature and field) and penetration depth on comparable composition samples gave some disagreement as to whether there was fully gapped/nodal behavior in the under-/overdoped materials respectively. The present work shows that the measured behavior of the specific heat gamma (proportional to C/T as T->0, i. e. a measure of the electronic density of states at the Fermi energy) as a function of field approximately obeys gamma proportional to H**(0.5 +- 0.1), similar to the Volovik effect for nodal superconductors, for both the underdoped and the most overdoped Co samples. However, for the two overdoped compositions x=0.103 and 0.13, the low field (H < 10 T) data show a Volovik-like behavior of gamma proportional to H**(0.3-0.4), followed by an inflection point, followed at higher fields by gamma proportional to H**1. We argue that within the 2-band theory of superconductivity, an inflection point may occur if the interband coupling is dominant.

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arXiv2012

Direct measurement of the magnetic penetration depth by magnetic force microscopy

We present an experimental approach using magnetic force microscopy for measurements of the absolute value of the magnetic penetration depth $λ$ in superconductors. $λ$ is obtained in a simple and robust way without introducing any tip modeling procedure via direct comparison of the Meissner response curves for a material of interest to those measured on a reference sample. Using a well-characterized Nb film as a reference, we determine the absolute value of $λ$ in a Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$ single crystal and a MgB$_2$ thin film through a comparative experiment. Our apparatus features simultaneous loading of multiple samples, and allows straightforward measurement of the absolute value of $λ$ in superconducting thin film or single crystal samples.

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arXiv2012

A direct probe of the variability of Coulomb correlation in Fe-pnictide superconductors

We use core-valence-valence (CVV) Auger spectra to probe the Coulomb repulsion between holes in the valence band of Fe pnictide superconductors. By comparing the two-hole final state spectra to density functional theory calculations of the single particle density of states, we extract a measure of the electron correlations that exist in these systems. Our results show that the Coulomb repulsion is highly screened and can definitively be considered as weak. We also find that there are differences between the 1111 and 122 families and even a small variation as a function of the doping, x, in Ba(Fe1 xCox)2As2. We discuss how the values of the hole-hole Coulomb repulsion obtained from our study relate to the onsite Coulomb parameter "U" used in model and first principles calculations based on dynamical mean field theory, and establish an upper bound for its effective value. Our results impose stringent constraints on model based phase diagrams

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arXiv2012

Properties of Binary Transition-Metal Arsenides (TAs)

We present thermodynamic and transport properties of transition-metal (T) arsenides, TAs with T = Sc to Ni (3d), Zr, Nb, Ru (4d), Hf and Ta (5d). Characterization of these binaries is made with powder X-ray diffraction, temperature and field-dependent magnetization and resistivity, temperature-dependent heat capacity, Seebeck coefficient, and thermal conductivity. All binaries show metallic behavior except TaAs and RuAs. TaAs, NbAs, ScAs and ZrAs are diamagnetic, while CoAs, VAs, TiAs, NiAs and RuAs show approximately Pauli paramagnetic behavior. FeAs and CrAs undergo antiferromagnetic order below TN = 71 K and TN \approx 260 K, respectively. MnAs is a ferromagnet below TC = 317 K and undergoes hexagonal-orthorhombic-hexagonal transitions at TS = 317 K and 384 K, respectively. For TAs, Seebeck coefficients vary between + 40 uV/K and - 40 uV/K in the 2 K to 300 K range, whereas thermal conductivity values stay below 18 W/(m K). The Sommerfeld-coefficient γ are less than 10 mJ/(K2mol). At room temperature with application of 8 Tesla magnetic field, large positive magnetoresistance is found for TaAs (~25%), MnAs (~90%) and for NbAs (~75%).

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arXiv2012

High resolution x-ray scattering studies of structural phase transitions in Cr-doped BaFe2As2

We have performed high resolution x-ray scattering measurements on single crystal samples of Ba(Fe(1-x)Cr(x))2As2 (x = 0 to 0.335). These measurements examine the effect of Cr-doping on the high temperature tetragonal (I4/mmm) to low temperature orthorhombic (Fmmm) structural phase transition of the parent compound BaFe2As2. Increasing Cr concentration is found to suppress the structural transition temperature (Ts), and reduce the magnitude of the orthorhombic strain (delta). The doping dependence of the orthorhombic strain, combined with complementary measurements of the high temperature magnetic susceptibility, suggests the presence of a magnetostructural crossover at x ~ 0.05. In particular, this crossover appears to mark a shift from strong to weak orthorhombicity and from predominantly itinerant to localized magnetic behavior.

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