Fe-chalcogenide based superconductors
A collaborative research between the departments of correlated matter, chemical metal sciences, and the physics of quantum materials in the field of Fe-based chalcogenides resulted in an extensive and widespread insight into the intricate physics of these interesting compounds. In the following we briefly summarize a few important results achieved in the past few years [1-8].
Sahana Rößler*, Horst Borrmann, Ulrich Burkhardt, Wilder Carrillo-Cabrera, Deepa Kasinathan, Hans-Henning Klauss1, Cevriye Koz, Yuri Grin, Philipp Materne1, Kamal Mydeen, Michael Nicklas, Ulrich K. Rößler2, Helge Rosner, Marcus Schmidt, Walter Schnelle, Ulrich Schwarz, Oliver Stockert and Steffen Wirth*
Among the different families of Fe-based superconductors, FeCh (11-type chalcogenide, Ch = Se, Te) has the simplest crystal structure . The atomic pattern belongs to the tetragonal P4/nmm space group and consists of edge-sharing FeCh4 tetrahedra, which form layers orthogonal to the c-axis. The bulk superconducting transition temperature Tc of FeSe is about 8.5 K at ambient conditions with the superconducting properties being extremely sensitive to the amount of excess Fe. For instance, it has been reported that Fe1.01Se is superconducting, however Fe1.03Se is not . Intriguingly, only the superconducting Fe1.01Se undergoes a structural phase transition from a tetragonal to an orthorhombic phase at Ts = 87 K while the non-superconducting Fe1.03Se does not show a corresponding structural transition . Tc can become as high as 37 K by the application of hydrostatic pressure [12, 13]. This makes FeSe a member of the high-Tc class of superconductors. The antiferromagnetic fluctuations are found to be strongly increased near Tc, and applied pressure seems to enhance the spin fluctuations as well as the superconducting transition temperature. In addition, Tc of FeSe can also be increased by Te substitution of Se, up to a maximum of Tc = 15 K for Fe1+ySe0.5Te0.5 [14–16]. The bulk superconductivity disappears for higher Te substitution and no superconductivity has been found so far in bulk samples of the end-member Fe1+yTe.
Fe1+yTe is a non-stoichiometric compound which orders antiferromagnetically at low temperatures. Initial studies revealed that the structural and magnetic properties are extremely sensitive to the chemical composition. The PbO-type tetragonal crystal structure of the Fe1+yCh phases accommodate excess Fe within the crystallographic 2c sites . Theoretical studies focusing on Fe1+yTe suggest a +1 valence state with a relatively large local moment of 2.4 μB for the interstitial Fe . These interstitial moments are expected to couple magnetically to the moments of the Fe square substructure resulting in a complex magnetic order. Therefore, it is highly important to establish the homogeneity range of these binary chalcogenides and to determine their structure-property correlations.
Using the state-of-the-art characterization techniques available at the department of chemical metal sciences, we established the homogeneity range, crystal structure, and phase diagrams of Fe1+yTe , FeSe , and their ternary variants .
By examining the phase diagrams of different classes of the high-Tc superconductors, it becomes apparent that the structural distortion, antiferromagnetic order and superconductivity are intricately related. Therefore, the knowledge of the low-temperature crystal structure and the related magnetic properties has become an integral component in understating the superconductivity of the Fe-based superconductors. We performed extensive low-temperature powder x-ray diffraction experiments, both at ambient and high pressures, at the European Synchrotron Radiation Facility (ESRF).
These experiments revealed a unique phase diagram of Fe1+yTe, see Fig. 1 [2, 3]. For y < 0.11, the compounds undergo a transition from a tetragonal paramagnetic into a monoclinic, bicollinear antiferromagnetic (AFM) phase upon decreasing the temperature. A two-step structural and magnetic phase transition occurs within the compositional range 0.11 £ y £ 0.13. For y > 0.13, once again a single phase transition to an incommensurate antiferromagnetic (IC AFM) phase with orthorhombic structure was found.
Very similar properties can also be achieved by using external pressure as a tuning parameter on Fe1+yTe with low Fe-content. In Fig. 1, a temperature-pressure phase diagram of Fe1.08Te is plotted. Results of Ref. 20 is also presented in Fig. 1 for comparison. Further, in addition to an IC AFM phase, a symmetry-conserving tetragonal-tetragonal phase transition was identified from a change in the c/a ratio of the lattice parameters for pressures higher than 2 GPa. From the high-pressure magnetization measurements, this high pressure-low temperature tetragonal phase was found to be ferromagnetic [21, 22]. The close resemblance of the temperature-composition and the temperature-pressure phase diagrams suggests a strong magneto-elastic coupling between the magnetic and structural order parameters in Fe1+yTe.
A continuous phase transition found in Fe1+yTe (0.11 < y < 0.15) [3, 23–25] from a tetragonal paramagnetic to an orthorhombic IC AFM phase is rather unconventional. Based on phenomenological models, we were able to shown that this IC AFM order violates the weak Lifshitz condition  in the Landau theory of second order transitions. The presence of multiple Lifshitz invariants provides the mechanism to create multidimensional, twisted, and modulated solitonic phases, Fig. 2. In this case, a magnetic precursor state similar to those found in chiral helimagnets [27–29] can also be anticipated in Fe1+yTe for y > 0.11. The Fe-based materials are ideally suited for identifying such quasi-static magnetic states above the long-range ordering temperature, because Mössbauer spectroscopy can be used to observe the inhomogeneous spin state established in these liquid-like spin structures. In fact, a liquid-like magnetic precursor with quasi-static spin-order was found from significantly broadened Mössbauer spectra for compositions with y = 0.13 and 0.15 at temperatures above the antiferromagnetic transition TN, Fig. 2(e).
In the following, we summarize our investigations on FeSe single crystals [4, 7]. Unlike in the case of Fe1+yTe, the chemical composition of single-phase tetragonal Fe1+ySe is very close to 1:1. We investigated FeSe single crystals using scanning tunneling microcopy/spectroscopy (STM/S), magnetization, and electrical transport measurements. The results showed evidence for the existence of two new energy scales T * and T0 below Ts. Our results indicated that T * represents the onset of an incipient order associated with enhanced spin fluctuations. Static nucleation of this mode below a second temperature T0 appears to result in a coupling between electronic charge, orbital, and pocket degrees of freedom as this temperature is discernible in anomalies of transport data and STM spectra.
Our experiments were conducted on single crystalline FeSe grown by chemical vapor transport by taking a mixture of powdered FeSe and AlCl3 as a transport agent in the ratio 50:1. By this method, a layer-by-layer growth along the c-axis was achieved. A complete characterization proving a high quality of the crystals used here can be found in Ref. . The topography of an in situ cleaved FeSe single crystal revealed atomically flat and largely defect-free Se-terminated (001) surfaces, Fig. 3 . The tunneling spectra obtained on such clean surfaces are presented in Fig. 4. At 6 K, the tunneling spectrum displays a superconducting gap with a full-width at half-minimum of 4.76 mV, see Fig. 4 (b). We estimated the superconducting gap width, D(T ® 0) ≈ 2.19 meV, which is in close agreement with the values reported for FeSe single crystals [30, 31]. Upon increasing temperature up to 40 K, two features can be discerned from the temperature evolution of the tunneling spectra; (i) the superconducting gap and coherence peaks disappear at around 8–9 K, as expected from Tc ≈ 8.5 K. (ii) a gap-like suppression in the DOS at EF persists well above T > Tc, Fig. 4(a).
Further, the tunneling spectra at low temperatures are highly asymmetric. The spectra become more and more symmetric above T * ≈ 75 K, Fig 4(c), which we identify as an onset temperature of an incipient ordering mode.
The temperature T * is also associated with the enhancement of spin fluctuations as evidenced by the nuclear magnetic resonance (NMR) [12, 32, 33] and magnetic susceptibility measurements . From the Kohler’s scaling behavior of the magnetoresistance (Figs. 5 (a) and (b)), we inferred that the scattering rates for the charge carriers are anisotropic below T *. However, at a lower temperature T0 ≈ 30 K, Kohler’s scaling is recovered, indicating a gap opening below T0. The Hall effect measurements on FeSe were also in accordance with the above interpretation. In Fig. 5(c), the Hall resistivities, rxy(H), measured at different temperatures are presented. The initial Hall coefficient RH®0(T) = is given in Fig. 5(d). RH®0(T) did not show a significant change at Ts, but goes through a maximum at ≈ 75 K ≈ Ts, and subsequently becomes negative below 60 K. Note that it can be seen from Fig. 4(d) that, at T *, the system initiates to undergo a transition from a nearly compensated metal to a strongly electron dominated transport regime.
These results are consistent with the strong electron-hole asymmetry found in the tunneling spectra below 75 K.
The crossover phenomenon observed at T * signals the appearance of an incipient, but likely incomplete ordering mode. This is associated with an anisotropic evolution of the Fermi surface, as seen from the violation of Kohler’s rule. Below T0, both, the validity of Kohler’s rule as well as the direct visualization of a partial gap in the local DOS suggest that the fluctuations of the ordering mode become more static upon reducing the temperature. The opening up of the partial gap in the DOS points toward a scenario of a subtle onset of some type of charge or orbital density wave that breaks the translational invariance of the lattice.
The precise nature of the ordering found here is not known, yet. However, unidimensional charge modulations are recently confirmed in several compounds belonging to the family of high-Tc cuprates. This underlying tendency for a charge modulation in different families of high-temperature superconducting materials points towards a generic scenario, where both charge and spin fluctuations likely play a key role in the mechanisms favoring the superconductivity at high temperatures.
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1 Institut für Festkörperphysik, TU Dresden, 01062 Dresden, Germany
2 IFW Dresden, Postfach 270016, 01171 Dresden, Germany