FeSe Single Crystals Research Articles

Growth, Annealing Effects on Superconducting and Magnetic Properties, and Anisotropy of FeSe1-xTex (0.5≤x≤1) Single Crystals

Single crystals of FeSe_1-x_Te_x_ (0.5=<x=<1) have been grown by the Bridgman method. After annealing them at 400 deg for 100 h in vacuum, single crystals of x=0.5-0.9 have exhibited bulk superconductivity. Anisotropic properties of the electrical resistivity and upper critical field, H_c2_, have been investigated for the single-crystal FeSe_1-x_Te_x_ with x=0.6. It has been found that the in-plane resistivity, Rho_ab_, shows a metallic temperature-dependence, while the out-of-plane resistivity, Rho_c_, shows a broad maximum around 100 K. The resistivity ratio, Rho_ab_/Rho_c_, is 44 and 70 at 290 K and just above the superconducting transition temperature, T_c_, respectively. The anisotropic parameter, gamma= H_c2_^para^/H_c2_^perp^ (The para and perp indicate field directions parallel and perpendicular to the ab-plane, respectively.), is estimated as 2.7 just below T_c_.

Magnetoelasticity of Fe–Si single crystals

The tetragonal magnetostriction constant, (3/2)λ100, of Fe–Si single crystals was measured and was found to be structure dependent. Similar to that of Fe–Ge single crystals, (3/2)λ100 is positive in the single phase A2 regime, becomes negative in the single phase D03 regime, and changes from positive to negative between the two regimes. Short-range order in the A2 regime decreases the magnetostriction prior to the onset of long range order. In the single phase regions of both A2 and D03, thermal history does not show any obvious effect on the magnetostriction, contrary to that found for Fe–Ga alloys. However, in the regions of phase mixture involving A2, B2, and D03 phases, quenching pushes the change in magnetostriction from positive to negative to higher Si contents.

Low temperature amorphization and superconductivity in FeSe single crystals at high pressures

In this study, we report low temperature x-ray diffraction studies combined with electrical resistance measurements on single crystals of iron-based layered superconductor FeSe to a temperature of 10 K and a pressure of 44 GPa. The low temperature high pressure x-ray diffraction studies were performed using a synchrotron source and superconductivity at high pressure was studied using designer diamond anvils. At ambient temperature, the FeSe sample shows a phase transformation from a PbO-type tetragonal phase to a NiAs-type hexagonal phase at 10 ± 2 GPa. On cooling, a structural distortion from a PbO-type tetragonal phase to an orthorhombic Cmma phase is observed below 100 K. At a low temperature of 10 K, compression of the orthorhombic Cmma phase results in a gradual transformation to an amorphous phase above 15 GPa. The transformation to the amorphous phase is completed by 40 GPa at 10 K. A loss of superconductivity is observed in the amorphous phase and a dramatic change in the temperature behavior of electrical resistance indicates formation of a semiconducting state at high pressures and low temperatures. The formation of the amorphous phase is attributed to a kinetic hindrance to the growth of a hexagonal NiAs phase under high pressures and low temperatures.

Characterization of FeSe single crystals

Superconductivity at 8K was reported in β-FeSe with a tetragonal PbO structure but not in δ-FeSe with a hexagonal NiAs structure. The superconducting transition temperature was then increased up to 37K (on-set) by high pressure on polycrystalline samples. We report a study of the characterization on single crystals of FeSe superconductors. FeSe single crystals were grown using two methods, simple cooling from stoichiometric melt and flux using NaCl/KCl. While a ball-shaped aggregation composed of thick FeSe crystals was grown from the stoichiometric melt, thin hexagonal-shaped crystals with a size up to 1mm were grown using the NaCl/KCl flux. The two different crystals indicated superconductivity around at 8K in spite of their hexagonal shape, suggesting a non-superconducting compound with the NiAs-type hexagonal structure. According to the analyses using an X-ray diffractometer and a scanning electron microscope, the crystals contain microcracks that could be caused by stress. We assume that the hexagonal-shaped crystals grown at high temperature may turn partly to tetragonal β-FeSe at the β–δ structure transition temperature.

Photoemission spectroscopy across the semiconductor-to-metal transition in FeSi

High-resolution angle-resolved photoemission spectroscopy (ARPES) was performed on high-purity FeSi single crystals. Since a high quality of the surface is obligatory to obtain reliably the correct band structure, special care was taken for the in situ preparation. Comparison of the experimental data with band structure calculations in the local density approximation shows that the self-energy resulting from electronic correlation effects leads to a strong renormalization of the energy bands in the vicinity of the Fermi energy. Temperature-dependent ARPES investigations reveal that at elevated temperatures the self-energy is strongly -dependent due to scattering of thermally excited particles and holes near the gap. From the ARPES results, it is obvious that the physical properties of FeSi are due to the high DOS at the edge of the renormalized band gap, but are not determined by a Kondo scenario.

Tracing the Goss Orientation during Deformation and Annealing of an FeSi Single Crystal

A Goss-oriented single crystal was cold rolled up to 89 % thickness reduction, and subsequently annealed at 550°C or 850°C. During deformation most of the initially Goss-oriented material rotated into the two symmetrical {111}&lt;112&gt; orientations. In addition, Goss regions were observed related to microbands or microshear bands. Goss regions in microshear bands formed during straining, whereas Goss regions between microbands were retained from the initial Goss orientation. The recrystallisation texture for annealing temperatures of both 550°C and 850°C is characterised by a Goss texture. However, the origin of the Goss recrystallisation nuclei appeared to be different for the different annealing conditions. In the material annealed at 550°C, the Goss texture originated from the Goss regions in the microshear bands. In contrast, for an annealing temperature of 850°C, the Goss grains between the microbands are likely to form recrystallisation nuclei.

Retention of the Goss orientation between microbands during cold rolling of an Fe3%Si single crystal

An FeSi single crystal with an initial {1 1 0}〈0 0 1〉 orientation, also referred to as Goss orientation, was cold rolled up to a thickness reduction of 89%. Most of the crystal volume rotated into the two symmetrical {1 1 1}〈1 1 2〉 orientations. However, a weak Goss component remained in the highly strained material, even though the Goss orientation is mechanically unstable under plane strain loading. Two types of Goss-oriented regions were discernable in the material subjected to 89% reduction. It appeared that these two types of Goss regions have different origins. Goss grains that were found aligned in shear bands form during straining. A second type of Goss region was found between microbands where the initial Goss orientation was retained.

Microstructure and Texture of Shear Bands in Cold Rolled Silicon Steel Single Crystals of Goss Orientation

An initially Goss-oriented ({110}&lt;001&gt; preferred crystal orientation) FeSi single crystal was cold rolled up to 89 % reduction in thickness. The microstructure and texture of shear bands, which develop at strains higher than 70 %, were investigated by the EBSD (electron backscatter diffraction) technique. The texture components within and ouside of the shear bands are the two symmetrical {111}&lt;112&gt; orientations and the {110}&lt;001&gt; orientation. We conclude that crystallographic slip is the deformation mechanism that is active both within and outside of the shear bands.

Re-evaluation of the Ibe–Lücke growth selection experiment in a Fe–Si single crystal

Selective growth effects during primary recrystallization have been widely discussed but so far no convincing proof has been provided. In the search for a more fundamental understanding of texture formation during the growth stage of static recrystallization, the historic growth experiment by Ibe and Lücke was re-evaluated by making use of modern electron backscattering diffraction equipment. Moreover, the non-random character of the nucleation texture was considered and the misorientations between the recrystallised grains and the single crystal matrix were examined in the 3D Rodrigues–Frank misorientation space. It was seen that the volume fraction of misorientations around the exact 〈1 1 0〉26.5° misorientation increased by a factor of 10 during growth. This is believed to provide unambiguous evidence for the occurrence of selective growth during the recrystallization process under consideration.

A new look at the atomic level virial stress: on continuum-molecular system equivalence

The virial stress is the most commonly used definition of stress in discrete particle systems. This quantity includes two parts. The first part depends on the mass and velocity (or, in some versions, the fluctuation part of the velocity) of atomic particles, reflecting an assertion that mass transfer causes mechanical stress to be applied on stationary spatial surfaces external to an atomic‐particle system. The second part depends on interatomic forces and atomic positions, providing a continuum measure for the internal mechanical interactions between particles. Historic derivations of the virial stress include generalization from the virial theorem of Clausius (1870) for gas pressure and solution of the spatial equation of balance of momentum. The virial stress is stress‐like a measure for momentum change in space. This paper shows that, contrary to the generally accepted view, the virial stress is not a measure for mechanical force between material points and cannot be regarded as a measure for mechanical stress in any sense. The lack of physical significance is both at the individual atom level in a time‐resolved sense and at the system level in a statistical sense. It is demonstrated that the interatomic force term alone is a valid stress measure and can be identified with the Cauchy stress. The proof in this paper consists of two parts. First, for the simple conditions of rigid translation, uniform tension and tension with thermal oscillations, the virial stress yields clearly erroneous interpretations of stress. Second, the conceptual flaw in the generalization from the virial theorem for gas pressure to stress and the confusion over spatial and material equations of balance of momentum in theoretical derivations of the virial stress that led to its erroneous acceptance as the Cauchy stress are pointed out. Interpretation of the virial stress as a measure for mechanical force violates balance of momentum and is inconsistent with the basic definition of stress. The versions of the virial‐stress formula that involve total particle velocity and the thermal fluctuation part of the velocity are demonstrated to be measures of spatial momentum flow relative to, respectively, a fixed reference frame and a moving frame with a velocity equal to the part of particle velocity not included in the virial formula. To further illustrate the irrelevance of mass transfer to the evaluation of stress, an equivalent continuum (EC) for dynamically deforming atomistic particle systems is defined. The equivalence of the continuum to discrete atomic systems includes (i) preservation of linear and angular momenta, (ii) conservation of internal, external and inertial work rates, and (iii) conservation of mass. This equivalence allows fields of work‐ and momentum‐preserving Cauchy stress, surface traction, body force and deformation to be determined. The resulting stress field depends only on interatomic forces, providing an independent proof that as a measure for internal material interaction stress is independent of kinetic energy or mass transfer.

AFM and SEM observation on mechanism of fatigue crack growth in an Fe-Si single crystal

Fatigue crack growth tests are carried out on sheets of an Fe-3.2% Si single crystal with a crystallographic orientation appropriate for striation formation. The behaviour of slip near a crack tip during the loading and unloading parts of a fatigue cycle is observed using an Atomic Force Microscope and a Scanning Electron Microscope. The fracture surfaces are also analysed with an AFM and an SEM. The mechanism of fatigue crack growth is discussed based on the observations, and a fundamental kinematic model for fatigue crack growth is proposed. The model gives a reasonable explanation for both the crack growth and striation formation.

Anodic dissolution of the (100) and (110) faces of iron monosilicide in a sulfuric acid electrolyte

Using cyclic voltammetry combined with an atomic absorption analysis, it is shown that, under anodic polarization of an FeSi single crystal in 0.5 M H2SO4, the metal-enriched (100) face dissolves faster. Fluoride-containing compounds promote anodic process due to dissolution of the surface layer of SiO2 and Si

Tunneling spectroscopy on the correlation effects in FeSi

We report for the first time on temperature-dependent spectroscopy of charge excitations in FeSi single crystals probed with a low-temperature STM between 4 and 300 K. Below ≈200 K, two peaks start to emerge in the differential conductance (d I/d V) on either side of the Fermi level, which grow with decreasing temperature and are separated by a (pseudo-)gap of ≈50 meV. We ascribe this change in the tunneling spectra to the formation of quasiparticle density of states caused by d-electron correlations. We find good agreement with photoemission spectroscopy since both methods are sensitive to the correlated d-electron density of states (DOS). Our results are also consistent with optical reflectivity data and Raman spectroscopy which, in contrast, are sensitive to the conduction (c) electron DOS.

Tunneling spectroscopy on the correlation effects in FeSi

We have performed electron tunneling spectroscopy on FeSi single crystals in the temperature range 4--300 K by using a scanning tunneling microscope. The differential conductance $(dI/dV),$ when corrected for Schottky barrier effects, exhibits two strongly temperature-dependent peaks on either side of the Fermi level that emerge below $\ensuremath{\approx}200$ K and that are separated by a (pseudo)gap of $\ensuremath{\approx}50$ meV. Our observations can be ascribed to the formation of quasiparticle density of states caused by d-electron correlation. The tunneling spectra are in good agreement with photoemission spectroscopy as both techniques probe the correlated d-electron density of states (DOS). Our results are also consistent with optical reflectivity data and Raman spectroscopy, which, in contrast, are sensitive to the conduction $(c)$ electron DOS.

Surface segregation on Fe–Si single crystals: the effect of crystallography

The kinetics of solute segregation was measured at the (001), (0 1 14), (017), (015), (014) and (013) surfaces of an Fe–6 at% Si alloy at 873 K. Individual stages of segregation kinetics can be explained on the basis of mutual interaction among the principal segregants (silicon, nitrogen, sulfur and potassium). Strong attractive interaction between silicon and nitrogen results in the formation of stable Si x N y two-dimensional surface compounds at all surfaces studied including (001). Formation of these Si–N bonds is accompanied by a shift in the kinetic energy of the Si LVV Auger peak. Analysing the segregation behaviour of individual surfaces indicates that it can be considered as a combination of the two principal (001) and (013) planes.

Low-temperature transport, thermodynamic, and optical properties of FeSi

We present a comprehensive series of electrical transport (conductivity, magnetoresistance, and Hall effect), thermodynamic (specific heat, magnetic susceptibility, and magnetization), and optical (reflectivity) measurements in varying temperature ranges between 0.05 and 330 K on high-quality FeSi single crystals grown by vapor transport. The entire set of data can consistently be described with the usual relations for a (compensated $n$ type) semiconductor if an unconventional band structure is assumed. Compared to the results of mean-field band-structure calculations, the height of the peaks in the total density of states around the energy gap is considerably enhanced, implying enhanced effective masses. Most likely correlation effects are the source of these features. At very low temperatures we encounter metallic behavior. A low concentration of correlated itinerant charge carriers coexists with interacting magnetic moments.

Band parameters of FeSi single crystals determined by magnetic measurements

The results of magnetization and magnetic-susceptibility measurements on FeSi single crystals are presented. The temperature dependence of the magnetic susceptibility in the range 5--300 K could be explained by contribution from the temperature-dependent parts due to paramagnetic centers and due to the carriers excited thermally in the intrinsic conductivity region. It is shown that the contribution to magnetic susceptibility of FeSi from single-occupied Anderson localized states is negligible. The values of the Curie-Weiss temperatures, an energy gap, as well as density-of-states effective mass were estimated.

Infrared spectroscopic study of phonons coupled to charge excitations in FeSi

From an investigation of the optical conductivity of FeSi single crystals using Fourier-transform infrared spectroscopy in the frequency range from 30 to 20 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ we conclude that the transverse effective charge of the Fe and Si ions is approximately 4e. Of the five optical phonons that are allowed by symmetry we observe only four, three of which have a Fano line shape presumably resulting from an interaction of these modes with the electronic continuum. We show that the large oscillator strength of the phonons results from a relatively weak coupling (\ensuremath{\lambda}\ensuremath{\approx}0.1) of the lattice degrees of freedom to an electronic resonance above the semiconductor gap, which is also responsible for the large electronic polarizability (${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\infty}}}$\ensuremath{\approx}200) of the medium.

Magnetic effects in transverse elastic constants of bcc Fe(Si)

We report measurements of the transverse elastic constants C′ = (C11 - C12)/2 and C44 of Fe(Si) single crystals from room temperature to T/TC ≌ 1.2 obtained from ultrasonic velocities determined by a pulse superposition method. Results are interpreted assuming that the elastic constants are the sum of an intrinsic magnetic contribution, proportional to the near-neighbour magnetic correlation function, and a ‘non-magnetic’ contribution (including indirect magnetostrictive contributions), approximated by a function of the lattice parameter only.

Magnetic properties and gap formation in FeSi

Magnetisation, magnetic susceptibility, resistivity, optical-reflectivity and point-contact tunnelling measurements were used to study an FeSi single crystal. Magnetisation measurements revealed a ferromagnetic contribution which is more pronounced below 100 K. Magnetic susceptibility, resistivity, far-infrared and point-contact data are consistent with the characterisation of FeSi as a narrow-gap semiconductor or Kondo insulator. Fits to our transport data suggest the formation of a gap at about Eg ≈ 650 K and a fit to our susceptibility data indicates a gap at about Eg ≈ 1200 K. The point-contact gap is filled at about 60 K. Optical spectra show a significant reduction of the low frequency reflectivity below 200 K.