Se Single Crystals Research Articles

Solution growth of chalcopyrite compounds single crystal

I–III–VI2 chalcopyrite compound Cu(Inx, Ga1−x)Se2 (CIGS) single crystal were successfully grown by traveling heater method (THM). The powder X-ray diffraction (XRD) pattern of the CIGS showed preferred orientations of (112), (220) and (312) planes, confirming the chalcopyrite structure. In Raman spectra, the A1 mode peaks expected for CIGS were observed, and no secondary phases were observed. The full-width at half-maximum (FWHM) of the X-ray rocking curve (XRC) for the (112) oriented CIGS single crystal is 103 arcsec. The composition of the CIGS single crystal was homogeneous and the stoichiometric ratio of CIGS was found to be slightly Cu-poor, In-rich, Ga-rich and Se-poor. The good-quality single-phase CIGS single crystals can be obtained from these results.

Synthesis and characterization of the layered iron-selenideNa0.8Fe1.6Se2

An iron-selenide Na0.8Fe1.6Se2 single crystal has been successfully synthesized using a self-flux method. The electrical resistivity measurement shows that this material exhibits semiconducting behavior in the whole temperature range, with an anomalous increment of resistivity at T-s similar to 595 K. By varying the concentrations of Na and Fe, a small volume of superconducting phase could be achieved with a critical temperature of T-c similar to 34 K. Structural characterization shows that, similarly to K0.8Fe1.6Se2, the Na0.8Fe1.6Se2 phase exhibits clear superstructure with a modulation wave vector of q = (3/5,1/5,0) caused by the Fe-vacancy order within the a-b plane.

Anisotropic characteristics and critical behaviors in Mn doped K0.8Fe2Se2 single crystal

We report the anisotropic superconducting properties of nominal K0.8Fe1.97Mn0.03Se2 single crystal. The phase separation is characterized by X-ray diffraction and scanning electron microscopy techniques. The experimental crystal has three superconducting transitions at Tc1mag=31.9K for the first, Tc2mag=36.8K for second, Tc3mag=42.5K for third superconducting phase as seen from magnetic susceptibility measurements. We can discriminate the anisotropic upper critical fields between the second and third superconducting phase by magnetotransport measurements. The upper critical field at T=0K are estimated to be μ0Hc2∥ab(0)=137.4T and μ0Hc2∥c(0)=51.8T for the second, μ0Hc2∥ab(0)=178.4T and μ0Hc2∥c(0)=52.6T for third superconducting phase, respectively. We also discuss the vortex pinning in framework of thermally activated flux flow model, by which the pinning potential U0 of the second superconducting phase is derived to be U0∥c=5.1×103K and U0∥ab=6.1×103K at μ0H=0.25T, respectively. Finally, we will evaluate the total critical current density, which shows moderate values Jc(0)∼103−104A/cm2 at T=3K.

Dynamics and mechanism of oxygen annealing in Fe1+yTe0.6Se0.4 single crystal

Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K. On the other hand, the presence of excess Fe is almost unavoidable in Fe(Te,Se) single crystals, which hinders the appearance of bulk superconductivity and causes strong controversies over its fundamental properties. Here we report a Systematical study of O2-annealing dynamics in Fe1+yTe1−xSex by controlling the amount of O2, annealing temperature, and time. Bulk superconductivity can be gradually induced by increasing the amount of O2 and annealing time at suitable temperatures. The optimally annealed crystals can be easily obtained by annealing with ~1.5% molar ratio of oxygen at 400°C for more than 1 hour. Superconductivity was witnessed to evolve mainly from the edge of the crystal to the central part. After the optimal annealing, the complete removal of excess Fe was demonstrated via STM measurements. Some fundamental properties were recharacterized and compared with those of as-grown crystals to discuss the influence of excess Fe.

Vortex creep down to 0.3 K in superconducting Fe(Te,Se) single crystals

We report on a study of the vortex creep in Fe${}_{1+\ensuremath{\delta}}($Te${}_{x}$Se${}_{1\ensuremath{-}x}$) single crystals ($x=0.5$ and $0.4$) down to 0.28 K ($\ensuremath{\sim}{T}_{c}/50$) and up to ${\ensuremath{\mu}}_{0}{H}_{a}=2$T. The relaxation of the current density [$J(t)$] has been measured during 20 hours and the decay of $J(t)$ can be well described by a $J(t)\ensuremath{\propto}{[ln(t/{t}_{0})]}^{\ensuremath{-}1/\ensuremath{\mu}}$ law. We show that the relaxation exponent $\ensuremath{\mu}$ tends towards 0 for $T<2$K and ${\ensuremath{\mu}}_{0}{H}_{a}<0.1$T [i.e. $J(t)\ensuremath{\rightarrow}{({t}_{0}/t)}^{\ensuremath{\alpha}}$] and increases for increasing $T$ and/or ${H}_{a}$. Our measurements strongly suggest that the logarithmic creep rate $R=\ensuremath{-}dln(J)/dln(t)$ remains finite at zero temperature (${R}_{|T\ensuremath{\rightarrow}0}\ensuremath{\rightarrow}2$%) and hence that quantum creep plays a dominant role in the relaxation process at low temperature. A maximum is observed in both the temperature and field dependence of $R(t=100\text{s},T,{H}_{a}),$ which can be associated to a crossover from a single vortex (one-dimensional) to a bundle (three-dimensional) creep regime.

Growth and physical characterization of AgGa1−xInxSe2 (x=0.5) single crystals grown by modified vertical Bridgman method

AgGa1−xInxSe2 (x=0.5) polycrystalline material was successfully synthesised from high purity elements by the melt and temperature oscillation method. Large size and crack-free AgGa0.5In0.5Se2 single crystals were grown using a double wall quartz ampoule with accelerated crucible rotation technique (ACRT) by modified vertical Bridgman method. The unit cell parameters were confirmed by single crystal X-ray diffraction analysis. The composition of AgGa0.5In0.5Se2 was measured using energy dispersive spectrometry (EDS). The insignificant change in atomic percentages of Ag, Ga, In and Se along the ingot further reveals that the composition throughout its length is fairly homogeneous. The transmittance spectra of AgGa0.5In0.5Se2 single crystal was achieved in the NIR region and high transmittance of the crystals in the mid-IR region was revealed. The absorption edge of the material is near 881nm and the optical band gap energy is 1.4eV. Thermal property of AgGa0.5In0.5Se2 has been studied using Differential scanning calorimetry (DSC) technique and it confirms that the increase in Indium content reduces the super-cooling temperature. Electrical property is measured using Hall Effect measurements and it confirms the n-type semiconducting nature.

Physical properties of KxNi2−ySe2 single crystals

We have synthesized K0.95(1)Ni1.86(2)Se2 single crystals. The single crystals contain K and Ni deficiencies not observed in KNi2Se2 polycrystals. Unlike KNi2Se2 polycrystals, the superconductivity is absent in single crystals. The detailed physical property study indicates that the K0.95Ni1.86Se2 single crystals exhibit heavy-fermion-like characteristics. The transition to a heavy fermion state below T ∼ 30 K results in an enhancement of the electron-like carrier density whereas the magnetic susceptibility shows little anisotropy and suggests the presence of both itinerant and localized Ni orbitals.

Temperature Dependence of $\sqrt{2} \times \sqrt{2}$ Phase in Superconducting K0.8Fe1.6Se2 Single Crystal

We report the structural phase transition properties of the newly discovered K0.8Fe1.6Se2 superconductor (T c =31.8 K) using synchrotron single-crystal X-ray diffraction. The basic structure of the sample at room temperature is found to be tetragonal ThCr2Si2-type, modulated by a vacancy ordering induced superlattice structure together with a coexisting minority phase having a $\sqrt{2} \times \sqrt{2}$ ordering. At 520 K, the reflections corresponding to the $\sqrt{2} \times \sqrt{2}$ phase merge with the parent tetragonal phase. The superlattice peaks corresponding to the vacancy ordering disappear at 580 K, indicating an order-disorder phase transition at this temperature.

Resonant photoemission spectroscopy of Cu(InGa)Se2 materials for solar cells

The electron structure of CuIn1 − x Ga x Se2 single crystals is determined via resonant photoemis-sion and the main regularities of its transformation upon varying concentration x from 0 to 1 are established. The dependence of the shape of valence band spectra on the photon energy is studied. Integral photoemission intensities are shown to be determined by atomic photoionization cross sections. Processes of the direct and two-step creation of photoelectrons accompanying photoemission and the participation of internal states in the spectra of electrons from valence bands are studied. Two-hole final states in photoemission are obtained upon threshold excitation of the Cu 2p level. The strong interaction of holes leads to the multiplet splitting of these states. Partial densities of the components’ states are determined using the energy dependence of atomic photoionization cross sections.

Vortex phase transition and isotropic flux dynamics in K0.8Fe2Se2 single crystal lightly doped with Mn

We have applied ac susceptibility measurements to study the vortex dynamics in a high quality K0.79Fe1.86Mn0.01Se2 single crystal with applied magnetic field both parallel and perpendicular to the c-axis. The irreversibility line has been determined in the frequency range of 133 Hz–9777 Hz which can be well fitted using Hirr=H0(1−T/Tc)β with β=1.49 for H∥c and β=1.63 for H∥ab. The field dependence of the activation energy U0 is derived in the framework of thermally activated flux creep theory, yielding a power law dependence ∼Hn with n≈−0.58 for H below 0.75 T and n≈−1 for H above 0.75 T. The derived activation energy U0 shows nearly isotropic behavior for both field directions and high values reaching 105 K at low fields. This observation points towards very strong intrinsic pinning in the investigated material, thus making KFeSe-systems promising for technical applications.

Diffuse phase transition from the superionic to non-superionic state in Cu1.8Se single crystal

The phase transition from the superionic to non-superionic state in a Cu1.8Se single crystal is studied by neutron diffraction. In the superionic state, a diffuse halo related to the disordering of the cation subsystem is observed. It is established that the phase transition from the superionic to non-superionic state is the diffuse first-order phase transition occurring in the temperature range 250–180 K.

Upper fields and critical current density of K0.58Fe1.56Se2 single crystals grown by one step technique

Single crystals of K0.58Fe1.56Se2 were successfully synthesized by a new single step process with the onset superconducting transition temperature 31.9K. The X-ray diffraction patterns suggest that they have high crystalline quality and c-axis orientation. A possible modulation structure of Fe–vacancy along c-axis was observed. The upper critical field has been determined with the magnetic field along ab-plane and c-axis, yielding an anisotropy of about 3.3. It has also been shown that the critical current density of the K0.58Fe1.56Se2 is about 1.7×104A/cm2 at 5K.

Temperature-dependent electrical resistivity, space-charge-limited current and photoconductivity of Ga0.75In0.25Se single crystals

Dark electrical resistivity, space-charge-limited (SCL) current and photoconductivity measurements were carried out on Ga0.75In0.25Se single crystals. Analysis of the dark resistivity measurements revealed the presence of one level with activation energy of 0.10eV. Current–voltage characteristics showed that both ohmic and SCL characters exhibit in 180–300K range. Analysis of the experimental data in the SCL region resulted with a trap level at 0.11eV above the valence band. Photoconductivity measurements were performed at different light intensities in the temperature range of 150–300K. Behavior of the recombination mechanism in the crystal was brought out as sublinear recombination from the dependence of photocurrent on illumination intensity. Moreover, obtained activation energies were compared with the results of other experimental techniques applied to Ga0.75In0.25Se crystals in literature.

Annealing effects on superconductivity in Rb0.81Fe1.72Se2 single crystal

The annealing effects on superconductivity in Rb0.81Fe1.72Se2 are investigated. For as-grown single crystal and the one annealed with furnace cooling, magnetic susceptibility exhibits a weak diamagnetic transition at 30 K. Consistently, the resistivity curve only displays a small drop at 30 K and an insulating behavior at lower temperatures. These features together with scanning electron microscope (SEM) data indicate that the superconducting phase exists as clusters in insulating matrix. As for the crystal annealed with fast quenching, resistivity results reveal a Tconset≈30 K and a Tczero≈10 K but the susceptibility does not show diamagnetic transition until the temperature is cooled down to 10 K. SEM results show that the surface of quenched sample is more homogeneous than the one annealed with furnace cooling. We argue that the annealing at high temperature makes the coexistence of superconducting and insulating phases becomes homogeneous. Then, the fast quenching preserves this state in which percolative superconducting paths could be easily formed. In comparison, during the furnace cooling, the temperature drops slowly and results in the segregation of the superconducting phase from the insulating phase into isolated clusters.

Temperature and excitation intensity tuned photoluminescence in Ga0.75In0.25Se crystals

Photoluminescence (PL) spectra of Ga0.75In0.25Se layered single crystals have been studied in the wavelength range of 580–670nm and temperature range of 7–59K. Two PL emission bands centered at 613nm (2.02eV, A-band) and 623nm (1.99eV, B-band) were revealed at T=7K. The excitation laser intensity dependence of the emission bands have been studied in the 0.06–1.40Wcm−2 range. Radiative transitions from shallow donor levels located at EA=0.11 and EB=0.15eV below the bottom of conduction band to single shallow acceptor level located at 0.01eV above the valence band are suggested to be responsible for the observed A- and B-bands. A simple model was proposed to interpret the recombination processes in Ga0.75In0.25Se single crystals.

Phase separation and magnetic order in the Tl0.75K0.25Fe1.86Se2 superconductor studied by Mössbauer spectroscopy

The results of a 57Fe Mössbauer spectroscopy study between 4.4 and 518.2K and in external magnetic fields of superconducting Tl0.75K0.25Fe1.86Se2 single crystals are reported. Evidence is provided for phase separation in the superconductor studied, into a majority magnetic and a minority diamagnetic/superconducting phase. It is shown that the majority magnetic phase orders antiferromagnetically with two different magnetic moments at 5.0K of 2.05(1)μB along the c-axis and 2.25(5)μB tilted away from the c-axis, and with the Néel temperature TN=517.2(2.1)K. The Debye temperatures of the magnetic and nonmagnetic phases are found to be, respectively, 206(3) and 211(2)K.

Low-temperature thermoluminescence in layered structured Ga0.75In0.25Se single crystals

Defect centers in Ga0.75In0.25Se single crystals have been studied performing the thermoluminescence measurements in the temperature range of 10–300K. The observed glow curves were analyzed using curve fitting, initial rise, and different heating rate methods to determine the activation energies of the defect centers. Thermal cleaning process has been applied to decompose the overlapped curves. Four defect centers with activation energies of 9, 45, 54 and 60meV have been found as a result of the analysis. The capture cross sections and attempt-to-escape frequencies of the defect centers were also found using the curve fitting method under the light of theoretical predictions. The first order kinetics for the observed glow curve was revealed from the consistency between the theoretical predictions for slow retrapping and experimental results. Another indication of negligible retrapping was the independency of peak position from concentration of carriers trapped in defect levels.

Magnetic ordering in semiconducting Tl0.53K0.47Fe1.64Se2 single crystals studied by Mössbauer spectroscopy

The results of a 57Fe Mössbauer spectroscopy study between 4.5 and 523.2 K and in external magnetic fields (up to 90 kOe) of semiconducting Tl0.53K0.47Fe1.64Se2 single crystals are reported. Evidence is provided for a possible phase separation into the magnetic majority and minority phases. It is demonstrated that the magnetic moments of the divalent Fe atoms located at the 16i site (space group I4/m) of the majority phase and of the minority phase are antiferromagnetically ordered, with the Néel temperature TN = 518.0(3.6) K. The magnetic moments at 5.0 K of 2.09(1) and 2.28(2) μB in these two phases are tilted from the crystallographic c axis by 18(1)∘ and 32(2)∘, respectively. The Debye temperature of Tl0.53K0.47Fe1.64Se2 is found to be 228(4) K.

Effect of γ-ray radiation on electrical properties of heat-treated Tb x Sn1 − x Se single crystals

The effect of γ-ray radiation on the electrical properties of heat-treated Tb0.01Sn0.99Se (sample 1) and Tb0.05Sn0.95Se (sample 2) samples is studied. It is found that, as a result of irradiation with γ-ray 1.25-MeV photons, the charge-carrier concentration decreases in the temperature range T = 77−200 K by 17 and 6.3% for samples 1 and 2, respectively. It is assumed that, in the course of irradiation with γ-ray photons, terbium impurity atoms are located between sites of the crystal lattice; in addition, Frenkel defects are formed.

Optical constants of layered structured Ga0.75In0.25Se crystals from the ellipsometric measurements

We have carried out the spectroscopic ellipsometry measurements on Ga0.75In0.25Se single crystals in the 1.2–6.0 eV spectral range at room temperature. The optical constants, real and imaginary parts of the dielectric function, refractive index and extinction coefficient, were found as a result of analysis of ellipsometric data. The critical point analysis of the second derivative spectra of the dielectric function revealed four interband transition structures with critical point energy values of 3.19, 3.53, 4.10 and 4.98 eV. The results of the analysis were compared with those of the ellipsometric studies performed on GaSe which is the main constituent of the Ga0.75In0.25Se crystal. The obtained critical point energies are in good agreement with the energies of the GaSe crystal reported in the literature.