Fe-chalcogenide based superconductors

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 Fe_1+yTe [3], FeSe [4], and their ternary variants [19].

By examining the phase diagrams of different classes of the high-T_c 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 Fe_1+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 Fe_1+yTe with low Fe-content. In Fig. 1, a temperature-pressure phase diagram of Fe_1.08Te 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 Fe_1+yTe.

A continuous phase transition found in Fe_1+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 [26] 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 Fe_1+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 T_N, Fig. 2(e).

In the following, we summarize our investigations on FeSe single crystals [4, 7]. Unlike in the case of Fe_1+yTe, the chemical composition of single-phase tetragonal Fe_1+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 T_s. 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. [4]. The topography of an in situ cleaved FeSe single crystal revealed atomi­cally flat and largely defect-free Se-terminated (001) surfaces, Fig. 3 [7]. 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 T_c ≈ 8.5 K. (ii) a gap-like suppression in the DOS at E_F persists well above T > T_c, 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 [7]. 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, r_xy(H), measured at different temperatures are presented. The initial Hall coefficient R_H®0(T) =  is given in Fig. 5(d). R_H®0(T) did not show a significant change at T_s, but goes through a maximum at  ≈ 75 K ≈ T_s, 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-T_c 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.