Role of Excess Fe in Pristine and Substituted Fe Chalcogenide Superconductors

Abstract

Fe chalcogenides : The discovery of superconductivity in Fe based compounds trig- gered an intense research activity in this field with significant importance given to material synthesis. As a result, numerous materials falling into four major classes and sharing similarities in physical properties were synthesized and investigated. In spite of subtle differences, all of them share many common features like crystal symmetry, magnetic ground state, close resemblance in phase diagram etc. Fe super- conductors are broadly classified into Fe pnictides (with Fe − pnictogen layer) and Fe chalcogenides (with Fe − chalcogen layer) in which the binary Fe chalcogenides possess the simplest crystal structure. The distinct magnetic and superconducting properties make them interesting candidates for research. Detailed study on such systems demand high quality single crystals. This thesis discusses single crystal growth and properties of Fe1+yTe1−xSex. Struc- tural, magnetic, superconducting and thermal properties of pristine and substituted compounds are explored. A characteristic feature associated with binary chalco- genides is the presence of excess Fe in the interstitial sites represented by y in the chemical formula. By fine tuning the composition, the effect of interstitial Fe on various physical properties can be analyzed. The current work deals with the influence of interstitial excess Fe on the structural, magnetic and superconducting properties of the parent compound Fe1+yTe and Se substituted Fe1+yTe1−xSex. The results are organized into eight chapters; an outline of each chapter is given below. Chapter 1 gives an introduction to the broad field of Fe superconductors. A de- tailed literature review providing comparison of Fe pnictides with chalcogenides is included. The background of the current work is discussed with reference to the im- portant aspects of crystal structure and its relation to the ordered ground states. An overview of the important theories on magnetic ordering and superconducting pair- ing is provided. In the later part, a generic phase diagram along with the individual phase diagrams of important systems are discussed. This is followed by a discus- sion of the characteristic properties of iron chalcogenides and different methods of bulk synthesis. The chapter is concluded with a note on the motivation behind the present work. Chapter 2 discusses the crystal growth techniques and experimental methods used in the present work. The basic working principles are briefly explained. Chapter 3 provides a detailed discussion of the single crystal growth procedure, its customization and basic characterization. Single crystals of all compositions un- der discussion are grown by a modified horizontal Bridgman method. Material preparation, growth parameters and overall temperature profile of crystal growth process are described. Single crystalline nature of the as-grown crystals is con- firmed with Laue scattering technique. All crystals show tetragonal symmetry at room temperature. The approximate crystal orientation is deduced by indexing the X-ray diffraction pattern of the cleaved crystals. The diffraction patterns exhibit a set of (00l) peaks. A detailed composition analysis is performed on the samples. The sample properties are very sensitive to composition and careful estimation is per- formed by conducting repeated measurements at multiple points on the samples under study. Chapter 4 deals with superconducting and magnetic properties of Fe1+yTe0.5Se0.5. Single crystals of two different Fe concentration, y=0.04 and 0.09 are grown in which the concentration of Se and Te are maintained close to 0.5. Among binary Fe chalcogenides, half substituted iron telluride shows the highest TC (15 K) at ambient pressure. Accordingly, this composition is chosen to evaluate the role of Fe concentration in modulating the superconducting behavior. Two different batches of both the samples are grown, one set containing small amounts of impurity phases and the other, representing a pure primary phase. Resistivity measurements performed on both compositions, y=0.04 and 0.09, show onset of superconductivity near 15 K. In the normal state above TC, the temperature derivative of resistivity dρ/dT changes from positive to negative as the excess Fe concentration rises. At higher Fe concentrations, a log 1/T divergence is discernible in the normal state. The contribution of interstitial Fe to superconductivity has been analyzed using magnetization measurement techniques. An increase in the width of superconducting transition is seen in all measurements as the Fe content increases. The superconducting volume fraction estimated from susceptibility data demonstrates that high concentration of Fe is not favorable to superconductivity. The upper and lower critical field are esti- mated from electric resistivity data (in applied magnetic field) and magnetization isotherms respectively. Comparison of the lower critical field between two compo- sitions strengthens the argument that higher excess Fe leads to suppression of super- conductivity. The second set of crystals with impurity phases reveals an anomalous magnetization peak near 125 K. The results from resistivity, DC magnetization and ac susceptibility are compared. Chapter 5 addresses the influence of excess Fe on the ordered ground state. The antiferromagnetic parent compound, Fe1+yTe single crystals, are also grown using the same procedure. It is proposed that excess Fe occupying the interstitial sites possess local moments which could interact with the magnetic phases. In an at- tempt to understand their magnetic properties in detail, single crystals are grown with y=0.06, 0.09, 0.11, 0.12, 0.13 and 0.15. Fe1+yTe undergoes magnetostructural transition at TN=67 K. As the concentration of Fe varies from 0.06 to 0.13, a marked suppression of TN occurs from 67 K to 56 K. Moreover, a single first order transi- tion is seen to split into two at the critical concentration, y=0.12. The derivative plot of magnetization and specific heat data clearly illustrate two well-separated peaks. The two transitions are denoted as TN=57 K and TS=46 K. TN here is identified as a second order transition and TS as a first order transition. The second order transi- tion is evident from the λ-like nature of the peak in the specific heat measurement. The first order transition is associated with a large thermal hysteresis in the heat- ing and cooling cycle. Raw data from the heat capacity calorimeter gives a clear hint towards the first order nature of TS. As the composition of Fe rises further, the multiple transitions subside and disappear. For higher concentration, y=0.15, a sin- gle continuous phase transition is observed. Impurity free, pure phase is noticed in most of the samples as evident in powder X-ray diffraction and bulk magneti- zation measurements. The thermal data of various compositions are analyzed and compared. Electrical resistivity data clearly reveals the shift in phase transition and the presence of multiple transitions. Unlike Fe1+yTe1−xSex, all compositions here display similar behavior above TN, irrespective of the concentration of excess Fe. Chapter 6 devotes special emphasis to the evolution of structural and magnetic properties of the critical composition, Fe1.12Te where multiple transitions are ob- served. The low temperature structure of the crystal is studied in detail using syn- chrotron powder X-ray diffraction. The data in the vicinity of the two transitions, TN and TS are carefully analyzed. The room temperature crystal structure belongs to tetragonal symmetry with P4/nmm space group, where it is paramagnetic. As the sample is cooled to just below TN, a magnetostructural transition occurs from tetragonal to orthorhombic space group Pmmn. Below TN, the XRD pattern of the tetragonal (200) peak splits into (200) and (020) representing an orthorhombic distortion. The second transition is observed at TS where the orthorhombic struc- ture undergoes a monoclinic distortion, to P21/m. Below TS, a mixed phase of or thorhombic and monoclinic structures are present. The powder diffraction studies are supplemented with thermodynamic measurements. From specific heat analy- sis, the different contributions and the change in entropy across the transitions are estimated. Linear thermal expansion study has confirmed the two structural transi- tions. The changes occurring in lattice parameters, bond distances, bond angles and unit cell volume as a function of temperature are calculated using powder pattern refinement. Synchrotron data, linear thermal expansion and thermodynamic mea- surement results all point to strong magnetostructural coupling in this material. A temperature-composition phase diagram is formulated using results obtained from different Fe compositions. Transition temperature is plotted as a function of excess Fe content, highlighting its role in determining the structural and magnetic phases in Fe1+yTe. Chapter 7 deals with the magnetic and superconducting properties of Se substi- tuted Fe1+yTe1−xSex. Single crystals are grown by carefully varying the concen- tration of Se from x=0.02 to 0.25 while keeping the nominal composition of excess Fe more or less same. In this work, focus is given to Fe-rich, selenium substituted compositions. The intention is to explore how Se substitution affects the multiple transitions observed in Fe1.12Te. At 2% Se substitution, the split peaks are evident with a slight shift in temperature. The temperature interval between the two tran- sitions decreases in comparison to the pristine compound. For further increases in Se concentration, instead of two well separated peaks, a weak broad hump is ob- served. For compositions with x >0.10, long range magnetic ordering is suppressed. As x increases above 0.15 the electrical resistivity drops indicating the onset of su- perconductivity. However, in the composition range 0.15 ≤ x ≤ 0.25, neither long range magnetic order nor bulk superconductivity is present. Alternately, weak magnetic transitions above the superconducting transition are visible. The transport and magnetic properties are similar to that observed in Fe1.09Te0.5Se0.5. By tuning the Se composition in Fe-rich samples, the magnetic and structural transitions, originally seen in the parent compound are suppressed. The emergence of superconductivity is also discussed. The last section of the chapter provides the modified phase diagram as a function of Se concentration, combining all compositions discussed in the thesis. This gives a detailed description of Fe chalcogenides in the composition range, x=0 to 0.5 with special emphasis on Fe rich samples. The different regions in the phases diagram describe the peculiar properties of Fe chalcogenides. Chapter 8 concludes the thesis with general conclusions pertaining to various observations made in the different chapters. Prospects for future work are briefly outlined.