Superconducting properties of iron chalcogenide thin films

Iron chalcogenides, binary FeSe, FeTe and ternary FeTe_xSe_1−x, FeTe_xS_1−x and FeTe:O_x, are the simplest compounds amongst the recently discovered iron-based superconductors. Thin films of iron chalcogenides present many attractive features that are covered in this review, such as: (i) easy fabrication and epitaxial growth on common single-crystal substrates; (ii) strong enhancement of superconducting transition temperature with respect to the bulk parent compounds (in FeTe_0.5Se_0.5, zero-resistance transition temperature T_c0^bulk = 13.5 K, but T_c0^film = 19 K on LaAlO₃ substrate); (iii) high critical current density (J_c ∼ 0.5 ×10⁶ A cm² at 4.2 K and 0 T for FeTe_0.5Se_0.5 film deposited on CaF₂, and similar values on flexible metallic substrates (Hastelloy tapes buffered by ion-beam assisted deposition) with a weak dependence on magnetic field; (iv) high upper critical field (∼50 T for FeTe_0.5Se_0.5, B_c2(0), with a low anisotropy, γ ∼ 2). These highlights explain why thin films of iron chalcogenides have been widely studied in recent years and are considered as promising materials for applications requiring high magnetic fields (20–50 T) and low temperatures (2–10 K).     

Superconducting properties of iron chalcogenide thin films

Abstract

Iron chalcogenides, binary FeSe, FeTe and ternary FeTe_xSe_1?x, FeTe_xS_1?x and FeTe:O_x, are the simplest compounds amongst the recently discovered iron-based superconductors. Thin films of iron chalcogenides present many attractive features that are covered in this review, such as: (i) easy fabrication and epitaxial growth on common single-crystal substrates; (ii) strong enhancement of superconducting transition temperature with respect to the bulk parent compounds (in FeTe_0.5Se_0.5, zero-resistance transition temperature T_c0^bulk = 13.5 K, but T_c0^film = 19 K on LaAlO₃ substrate); (iii) high critical current density (J_c ～ 0.5 ×10⁶ A cm² at 4.2 K and 0 T for FeTe_0.5Se_0.5 film deposited on CaF₂, and similar values on flexible metallic substrates (Hastelloy tapes buffered by ion-beam assisted deposition) with a weak dependence on magnetic field; (iv) high upper critical field (～50 T for FeTe_0.5Se_0.5, B_c2(0), with a low anisotropy, γ ～ 2). These highlights explain why thin films of iron chalcogenides have been widely studied in recent years and are considered as promising materials for applications requiring high magnetic fields (20–50 T) and low temperatures (2–10 K).     

O<Subscript>2</Subscript> Annealing Induced Superconductivity in FeTe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>: on the Origin of Superconductivity in FeTe Films

FeTe_1?x Se _x with x = 0 ～ 0.13 polycrystalline samples was fabricated by solid-state reaction and annealed in oxygen. The magnetic and transport measurements illustrated that neither the as-grown nor the O₂-annealed samples with x = 0.05 showed superconductivity. The as-grown samples with x = 0.07 ～ 0.09 also showed no superconductivity but became filamentary superconducting after the O₂ annealing. Significant enhancement of bulk superconductivity was achieved for the O₂-annealed FeTe_1?xSe _x with x = 0.11. X-ray photoelectron spectroscopy measurements illustrated that the change of the chemical valence of the elements before and after the O₂ annealing was not the main factor responsible for the occurrence of superconductivity. The superconducting transition was mainly caused by the suppression of antiferromagnetic ordering, due to the lattice shrinkage induced by the O₂ annealing. These results may clarify the existing debate on the origin of the superconductivity in FeTe thin film.     

Heat Capacity and Mössbauer Study of Self-Flux Grown FeTe Single Crystal

We report mainly the heat capacity and Mössbauer study of self-flux grown FeTe single crystal, which is a ground state compound of the Fe chalcogenides superconducting series i.e. FeTe_1?x(Se/S) _x The as grown FeTe single crystal is large enough to the tune of a few centimetres and the same crystallizes in tetragonal structure having space group of P4/nmm. FeTe shows the structural/magnetic phase transition at 70 K in both magnetic and resistivity measurements. Heat capacity measurement also confirms the coupled structural/magnetic transition at the same temperature. The Debye model fitting of low temperature (below 70 K) heat capacity exhibited Debye temperature (?? _D ) to be 324 K. Mössbauer spectra are performed at 300 and 5 K. The 300-K spectra showed two paramagnetic doublets and the 5-K spectra exhibited hyperfine magnetic sextet with an average hyperfine field of 10.6 Tesla matching with the results of Yoshikazu Mizuguchi et al.     

Inhomogeneous Electronic Structure of FeTe1−x Se x Revealed by X-ray Absorption Near Edge Structure

Ab initio Fe K-edge X-ray absorption near edge structure (XANES) calculations on FeTe_1?x Se _x , based on its crystallographic structure, are compared with experimental data. The calculated XANES spectra are inconsistent with the observed increase of spectral weight in the preedge region of the experimental spectra on the ternary system in which Te is substituted by Se. However, the weighted average of the XANES of FeTe and FeSe binaries reproduce well the observed trend in the experimental spectra of the ternary systems. This suggests that the FeTe_1?x Se _x is characterized by an inhomogeneous local electronic structure, resulting from a random-alloy like local atomic structure. The calculated unoccupied Fe d-density of states at the Fermi level increases monotonically with Te content unlike the observed change in T _c with Te content. T _c increases with increasing Te content up to x=0.5; however, the superconductivity is suppressed in the FeTe due to magnetic order, which is not considered in these calculations.     

Anomalous Impact of Hydrostatic Pressure on Superconductivity of Polycrystalline LaO<Subscript>0.5</Subscript>F<Subscript>0.5</Subscript>BiSe<Subscript>2</Subscript>

We report bulk superconductivity at 2.5 K in LaO_0.5F_0.5BiSe₂ compound through the DC magnetic susceptibility and electrical resistivity measurements. The synthesized LaO_0.5F_0.5BiSe₂ compound is crystallized in tetragonal structure with space group P4/nmm and Reitveld refined lattice parameters are a = 4.15(1) Å and c = 14.02(2) Å. The lower critical field of H _c1 = 40 Oe, at temperature 2 K is estimated through the low field magnetization measurements. The LaO_0.5F_0.5BiSe₂ compound showed metallic normal state electrical resistivity with residual resistivity value of 1.35 m Ω cm. The compound is a type-II superconductor, and the estimated H _c2(0) value obtained by WHH formula is above 20 kOe for 90 % ρ _n criteria. The superconducting transition temperature decreases with applied pressure till around 1.68 GPa and with further higher pressures a high- T _c phase emerges with possible onset T _c of above 5 K for 2.5 GPa.     

Superconductivity at 25 K under Hydrostatic Pressure for FeTe 0 . 5 Se 0 . 5 Superconductor

We report the impact of hydrostatic pressure on the superconductivity and normal-state resistivity of FeTe_0.5Se_0.5 superconductor. At the ambient pressure, the FeTe_0.5Se_0.5 compound shows the superconducting transition temperature \(T_{\mathrm {c}}^{\text {onset}} \) at above 13 K and \(T_{\mathrm {c}}^{\rho =0} \) at 11.5 K. We measure pressure-dependent resistivity from 250 to 5 K, which shows that the normal-state resistivity increases initially for the applied pressures of up to 0.55 GPa, and then the same is decreased monotonically with increasing pressure of up to 1.97 GPa. On the other hand, the superconducting transition temperatures ( \(T_{\mathrm {c}}^{\text {onset}} \) and \(T_{\mathrm {c}}^{\rho =0} )\) increase monotonically with increasing pressure. Namely the \(T_{\mathrm {c}}^{\text {onset}} \) increases from 13 to 25 K and \(T_{\mathrm {c}}^{\rho =0} \) from 11.5 to 20 K for the pressure range of 0–1.97 GPa. Our results suggest that superconductivity in this class of Fe-based compounds is very sensitive to pressure as the estimated pressure coefficient d T _c(onset)/dP is ～5.8 K/GPa. It may be suggested that the FeTe_0.5Se_0.5 superconductor is a strong electron-correlated system. The enhancement of T _c with applying pressure is mainly attributed to an increase of charge carriers at the Fermi surface.     

Synthesis and Superconductivity of New BiS2 Based Superconductor PrO0.5F0.5BiS2

We report synthesis and superconductivity at 3.7 K in PrO_0.5F_0.5BiS₂. The newly discovered material belongs to the layered sulfide based REO_0.5F_0.5BiS₂ compounds having a ZrCuSiAs-type structure. The bulk polycrystalline compound is synthesized by the vacuum encapsulation technique at 780 ^°C in a single step. Detailed structural analysis has shown that the as synthesized PrO_0.5F_0.5BiS₂ is crystallized in a tetragonal P4/nmm space group with lattice parameters a=4.015(5) Å, c=13.362(4) Å. Bulk superconductivity is observed in PrO_0.5F_0.5BiS₂ below 4 K from magnetic and transport measurements. Electrical transport measurements showed superconducting transition temperature (T _c ) onset at 3.7 K and T _c (ρ=0) at 3.1 K. The hump at T _c related to the superconducting transition is not observed in the heat capacity measurement and rather a Schottky-type anomaly is observed at below ～6 K. The compound is slightly semiconducting in a normal state. Isothermal magnetization (MH) exhibited typical type II behavior with a lower critical field (H _c1) of around 8 Oe.     

Seebeck Coefficient Measurement and Its Narrow Band Model Interpretation in FeTe<Subscript>0.5</Subscript>Se<Subscript>0.5</Subscript> Superconductor

We have measured Seebeck coefficient (S) of FeTe_0.5Se_0.5 superconducting sample from 10 to 300 K. The variation of Seebeck coefficient with temperature of this system was found to be very anomalous, and the overall experimental observation of the S(T) was studied in the outline of a narrow-band model. In high-temperature region, the Seebeck coefficient is almost independent of temperature. Further, from the study of high-temperature magnitude of S, sample undergoes a change in sign in the Seebeck coefficient, wherein, appearance of a negative peak around 22 K and subsequently, its Seebeck coefficient goes to zero in the superconducting transition temperature regime around 11 to 13 K. It is revealed that the bandwidth and a small asymmetry involved in narrow bands give a realistic explanation to the anomalous temperature dependence of Seebeck coefficient of FeTe_0.5Se_0.5 system.     

The Annealing Effects in the Iron-Based Superconductor FeTe0.8Se0.2 Prepared by the Self-Flux Method

FeTe_0.8Se_0.2 single crystals as-cast and post-annealed were prepared by the self-flux method. We have investigated the structural properties of samples by using the XRD, scanning electron microscope (SEM), energy dispersive X-ray (EDX), and magnetic techniques. The SEM results clearly demonstrate that Te ions are quite well substituted for Se ions in the FeSe lattice for the samples. From the XRD and EDX spectra of the both samples, it has been concluded that the post-annealing causes no change in the tetragonal structure of FeTe_0.8Se_0.2. According to M–H measurements, the perfect diamagnetism has been observed only in low field at 5 and 10 K temperatures. The trend of the magnetization versus temperature curves, measured under a magnetic field of 10 Oe, also support our conclusion about diamagnetic contribution in FeTe_0.8Se_0.2 single crystal explored in this study. The as-cast and post-annealed samples show the onset of diamagnetism at temperatures, \(T_{\mathrm {c.on}}^{\text {mag}}\) , 12.45 and 13.27 K, respectively. In addition, those curves indicate that the high field value and some impurities reveal ferromagnetic interactions.     

Superconductivity of FeSr2YCu2O6+δ

Resistivity measurements of polycrystalline FeSr₂YCu₂O_6+δ under magnetic fields up to 160 kOe were made to study the superconductivity of FeSr₂YCu₂O_6+δ in detail. The resistivity began to decrease at 64 K and dropped to zero at 38 K under zero magnetic field. The superconductivity in inter grain was affected by the magnetic field and zero resistivity was observed below 12 K under H=10 kOe. Above 20 kOe, the superconductivity in the grain began to be affected. Even with increasing magnetic field up to 160 kOe, zero resistivity was persisted below 10 K.     

Investigation of Anomalous Magnetic Transition in Pr0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxides Pr_0.5Sr_0.5CoO₃ were prepared by the sol-gel method. We mainly study the anomalous magnetic transition of Pr_0.5Sr_0.5CoO₃. We report the investigations of polycrystalline samples of the metallic ferromagnetic material Pr_0.5Sr_0.5CoO₃ through measurements of X-ray diffraction, the magnetization, and the resistivity. We found an unusual anomaly around T _A=120 K, much below the ferromagnetic transition (T _C=228 K). Further using the variable temperature X-ray diffraction and electron spin resonance (ESR) measurements, we found that as the temperature goes down, the crystal structure changes obviously at T _A=120 K. We show that this actually results in anomalous magnetic transition.     

Superconductivity in Layered CeO0.5F0.5BiS2

We report appearance of superconductivity in CeO_0.5F_0.5BiS₂. The bulk polycrystalline samples CeOBiS₂ and CeO_0.5F_0.5BiS₂ are synthesized by conventional solid state reaction route via vacuum encapsulation technique. Detailed structural analysis showed that the studied CeO_0.5F_0.5BiS₂ compound is crystallized in tetragonal P4/nmm space group with lattice parameters a=4.016(3) Å, c=13.604(2) Å. DC magnetization measurement (MT-curve) shows the ferromagnetic signal at the low temperature region. The superconductivity is established in CeO_0.5F_0.5BiS₂ at $T_{c}^{\mathrm{onset}}=2.5~\mbox{K}$ by electrical transport measurement. Under applied magnetic field, both T _c onset and T _c (ρ=0) decrease to lower temperatures and an upper critical field [H _c2(0)] above 1.2 Tesla is estimated. The results suggest coexistence of ferromagnetism and superconductivity for the CeO_0.5F_0.5BiS₂ sample.     

Superconductivity in Ca<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>FBiSe<Subscript>2</Subscript>

Through the measurement of resistivity, magnetic susceptibility, and Hall effect, we discovered a novel BiSe₂-based superconductor Ca_0.5La_0.5FBiSe₂ with T _c of 3.9 K. A strong diamagnetic signal below T _c in susceptibility χ(T) is observed indicating the bulk superconductivity. The negative Hall coefficient throughout the whole temperature regime implies the dominant electron-type carriers in the sample. Different to most of BiS₂-based compounds where superconductivity develops from a semiconducting-like normal state, its resistivity in the present compound exhibits a metallic behavior down to T _c . Together with the enhanced T _c , the metallic character of the normal state implies that the electronic structure of Ca_0.5La_0.5FBiSe₂ may be different to those in the other BiS₂-based compounds.     

Magnetic excitations in iron chalcogenide superconductors

Nuclear magnetic resonance and neutron scattering experiments in iron chalcogenide superconductors are reviewed to make a survey of the magnetic excitations in FeSe, FeSe_1−xTe_x and alkali-metal-doped A_xFe_2−ySe₂ (A = K, Rb, Cs, etc). In FeSe, the intimate relationship between the spin fluctuations and superconductivity can be seen universally for the variations in the off-stoichiometry, the Co-substitution and applied pressure. The isovalent compound FeTe has a magnetic ordering with different wave vector from that of other Fe-based magnetic materials. The transition temperature T_c of FeSe increases with Te substitution in FeSe_1−xTe_x with small x, and decreases in the vicinity of the end member FeTe. The spin fluctuations are drastically modified by the Te substitution. In the vicinity of the end member FeTe, the low-energy part of the spin fluctuation is dominated by the wave vector of the ordered phase of FeTe; however, the reduction of T_c shows that it does not support superconductivity. The presence of same wave vector as that of other Fe-based superconductors in FeSe_1−xTe_x and the observation of the resonance mode demonstrate that FeSe_1−xTe_x belongs to the same group as most of other Fe-based superconductors in the entire range of x, where superconductivity is mediated by the spin fluctuations whose wave vector is the same as the nesting vector between the hole pockets and the electron pockets. On the other hand, the spin fluctuations differ for alkali-metal-doped A_xFe_2−ySe₂ and FeSe or other Fe-based superconductors in their wave vector and strength in the low-energy part, most likely because of the different Fermi surfaces. The resonance mode with different wave vector suggests that A_xFe_2−ySe₂ has an exceptional superconducting symmetry among Fe-based superconductors.     

Characterization and Low Temperature Study of Iron Telluride Thin Films Upon Ageing and Oxygen Ion Irradiation

The occurrence of superconductivity in iron telluride thin films has been observed upon ageing. The superconducting transition is found to be very robust under an application of magnetic fields up to 10 T. Infrared and Raman spectroscopic characterization of the aged films reveal the formation of Fe_1+δTeO _x upon ageing. As an alternative method of introducing oxygen in FeTe thin film, oxygen ion irradiation has also been carried out It is found that the irradiated film becomes increasingly disordered and ultimately transforms to an amorphous phase upon increasing the irradiation dose. Investigation of electrical resistivity and optical reflectivity of the irradiated FeTe films indicates an interesting possibility of an ion irradiationinduced phase change memory material in analogy to the phase change characteristics of laserirradiated FeTe films.     

Zero resistance states in modified Y-Ba-Cu-O system with transition temperatures above 135 K

Superconducting transition of minor-dispersed phase in modified YBa₂Cu₃O_7?δ samples is found to occur at around 140K. The amount of this minor phase is enough to provide zero resistivity above 135 K. The measurements of the electrical resistivity indicated that the material is stable, thermally recyclable and reproducible. X-ray analysis of the sample with the highestT _c shows a major phase with perovskite-like structure witha=3·820(1) Å;b=3·873(1) Å andc=11·659(2) Å along with several unidentified weak peaks. Magnetic measurements confirmed the mixed-phase nature with diamagnetic transition temperatures at 137,91 and 86 K. The minor phase responsible for superconductivity with zero resistivity above 135 K is about 0·4% of the bulk and its nature is still unidentified. The details of the preparation and chemical modification process and the results are presented.     

Heat capacity, thermopower and magnetoresistance effects in multiferroic La0.5Bi0.5Mn0.5Fe0.5O3

We have carried out systematic investigations in perovskite multiferroic La_0.5Bi_0.5Mn_0.5Fe_0.5O₃ by means of X-ray diffraction, magnetisation, electrical resistivity, thermoelectric and heat capacity measurements. The magnetic behaviour of this composition is rather complex, though the magnetisation curve seems to be like a weak ferromagnetic material. However, there is no clear evidence of λ-anomaly in the heat capacity data down to 2 K, yet this behaviour corroborate the trends of semiconducting silicon below room temperature. The sensitivity of magnetic behaviour to the iron-manganese ratio is also demonstrated. In presence of an external field of 7 T, it exhibits a magnetoresistance of ?5 % at 130 K. The thermoelectric value increases linearly with decreasing temperature, and at room temperature the value is +85 μV/K, which is associated with the p-type polaronic conductivity.     

Magneto-transport and Magnetic Susceptibility of SmFeAsO1−x F x (x=0.0 and 0.20)

We report superconductivity in the SmFeAsO_1?x F _x for the x=0.2 system being synthesized using the single step solid state reaction route. Rietveld analysis of room temperature X-ray diffraction (XRD) data shows the studied samples, SmFeAsO_1?x F _x with x=0.0 and x=0.2, are crystallized in a single phase tetragonal structure with space group P4/nmm. The resistivity measurement shows superconductivity for the x=0.20 sample with T _c (onset) ～51.7 K. The upper critical field, [H _c2(0)] is estimated ～3770 kOe by Ginzburg–Landau (GL) theory. Broadening of superconducting transition in magnetotransport is studied through thermally activated flux flow in an applied field up to 130 kOe. The flux flow activation energy (U/k _B ) is estimated ～1215 K for 1 kOe field. Magnetic measurements exhibited bulk superconductivity with lower critical field (H _c1) of ～1.2 kOe at 2 K. In the normal state, the paramagnetic nature of compound confirms no trace of magnetic impurity, which orders ferromagnetically. AC susceptibility measurements have been carried out for SmFeAsO_0.80F_0.20 sample at various amplitude and frequencies of applied AC drive field. The intergranular critical current density (J _c ) is estimated. Specific heat [C _p (T)] measurement showed an anomaly at around 140 K due to the SDW ordering of Fe, followed by another peak at 5 K corresponding to the antiferromagnetic (AFM) ordering of Sm⁺³ ions in the SmFeAsO compound. Interestingly, the change in entropy (marked by the C _p transition height) at 5 K for Sm⁺³ AFM ordering is heavily reduced in the case of the superconducting SmFeAsO_0.80F_0.20 sample.     

Magnetic Order and Superconductivity in Eu1.5Ce0.5RuSr2Cu2O10

AC magnetic susceptibility, (T), and do resistivity, (T), measurements have been performed to study the superconductivity and magnetic properties of a polycrystalline Eu_1.5Ce_0.5RuSr₂Cu₂O₁₀. X-ray diffraction shows that the single-phase compound crystallized into a tetragonal symmetry of space group I4/mmm. Ru spin ordering and superconductivity at 75 and 37 K, respectively, were evident in (T). The magnetic responses associated with the Ru spin ordering were found to be strongly reduced by a weak applied field, indicating an antiferromagnetic character for the Ru moments. A two-step transition to the superconducting state was observed in (T), where an intra-granular transition at T _C1=37 K and an inter-granular transition at T _C2=20 K were clearly seen. These observations confirm the coexistence of superconductivity and magnetic order at low temperatures.