Theory of Normal State Pseudogap Behavior in FeSe$_{1}$$_{−}$ x Te x L. Craco 1 and M.S. Laad 2 $^{1}$Max-Planck-Institut f¨ ur Chemische Physik fester Stoffe, 01187 Dresden, Germany $^{2}$Institut f¨ ur Theoretische Physik, RWTH Aachen, 52056 Aachen, Germany (Dated: October 31, 2018) The normal state of the recently discovered Iron Selenide (FeSe)-based superconductors shows a range of inexplicable features. Along with bad-metallic resistivity, characteristic pseudogap features and proximity to insulating states, reminiscent of the underdoped high-T c cuprates, mark these sys- tems as strongly correlated non-Fermi Liquid metals. Here, using the first-principles LDA+DMFT method, we show how strong multi-orbital correlation-induced orbital-selective Mott-like physics leads to an orthogonality catastrophe underpinning these inexplicable incoherent features. Excel- lent agreement with a range of resistivity and Seebeck data strongly support our proposal. We discuss pseudogap regime microscopically, along with implications for the nature of the instability at lower T , and propose that related systems could be of use in thermoelectric devices. The Iron Selenides (FeSe), with or without Tellurium (Te) substitution, are the latest addition to a rapidly growing list of Fe-based unconventional superconduc- tors [1]. These latter systems are structurally simpler than their 1111-oxyarsenide counterparts [2]; they never- theless exhibit a host of very unusual physical responses. These are (by no means a complete list): ( i ) a linear- in- T dependence of the bad metallic normal state resis- tivity, ρ$_{dc}$ ( T ) ≃ ρ$_{0}$ + AT above T$_{c}$ , and a Te- and Cu- substitution induced metal-insulator transition (MIT) in FeSe$_{1}$ − $_{x}$Te$_{x}$ [3] and Fe$_{1}$ − $_{x}$Cu$_{x}$ Se [4]. ( ii ) The NMR relax- ation rate is distinctly of the non-Korringa form, and the static (uniform, q = 0) spin susceptibility, χ ( T ) ≃ T 1+ n with 0 < n < 1, in FeSe [5]. ( iii ) An almost non- existent Drude peak, with dominant incoherent features in the optical conductivity [6]. ( iv ) Incoherent pseudo- gap like low-energy features in angle-integrated photoe- mission (PES) [7]. Also, an ARPES study for the anti- ferromagnetic compositions shows that the AF ordering wave-vector, Q$_{AF}$ , is very different from that predicted by local-density-approximation (LDA) [8]. In particular, susceptibility and PES data show up the normal state pseudogap (PG) in FeSe$_{1}$ − $_{x}$Te$_{x}$ . Several of the above features are reminiscent of the high- T$_{c}$ cuprates in the under- to optimally doped regime, and, taken to- gether, defy an interpretation in terms of the Landau Fermi Liquid (LFL). These PG features must manifest themselves in other probes as well. In fact, recent nor- mal state resistivity and Seebeck data also exhibit PG features which closely correlate with each other [the PG in ρ$_{dc}$ ( T ), as seen in ( dρ/dT ) and ( d $^{2}$ρ/dT $^{2}$) near the PG scale T ∗ ≡ T$_{PG}$ , correlates well with the related anomaly in S ( T ) at the same T = T $^{∗}$] [9, 10]. Further, at low T , the thermopower does not show the S ( T ) ≃ aT behav- ior; the T dependence is distinctly slower than linear just above T$_{c}$ . Along with ρ$_{dc}$ ( T ) ≃ AT , this confirms that the normal state in FeSe$_{1}$ − $_{x}$Te$_{x}$ is not a LFL metal. In addition, S ( T ) at high- T flattens out [9], being well de- scribed by a Heikes-like law; this is exactly the behavior expected from a Hubbard-like model at high T . Finally, Since the basic LDA+DMFT formulation has been In earlier work, we have shown that the incoher- ent metal [12] as well as the Te-doping induced metal- insulator transition (MIT) [13] can be quantitatively un- derstood using LDA+DMFT with sizable d band MO correlations. Here, we extend these to characterize the PG phase in depth. Specifically, we show how the See- beck co-efficient (thermopower), S ( T ) is also quantita- tively described, and correlate the specific PG anomalies in both S ( T ) and ρ$_{dc}$ ( T ) with doping, x . Very good quantitative accord with extant data reinforces the ba- sic hypothesis about crucial role of strong correlations. To our best knowledge, this is a first attempt to uncover such features in an incoherent metal within LDA+DMFT for a real system. Finally, we show how the itinerant- localized duality (Mottness) makes for a high thermo- electric figure-of-merit ( ZT ) in such systems at low T , and suggest possibilities for thermoelectric applications using related systems. Taken together, the above also imply that the FeSe$_{1}$ − $_{x}$Te$_{x}$ are strongly correlated materials proximate to a Mott insulator. The microscopic electronic cor- relation processes underlying the emergence of an in- coherent pseudogapped metallic phase above T$_{c}$ ( T$_{N}$ ) in FeSe$_{1}$ − $_{x}$Te$_{x}$ , however, await a consistent theoretical understanding. Like the 1111-systems, Fe-selenides are multi-orbital (MO) systems with all five d bands cross- ing the Fermi energy ( E$_{F}$ ) in band (LDA) calculations. Given the global non-LFL features found experimen- tally, however, non-trivial extensions of the LDA to ac- count for dynamical correlation-driven incoherence and breakdown of LFL theory are mandatory. Here, using the state-of-the-art LDA+dynamical-mean-field-theory (LDA+DMFT), already used with good success [11] for a range of correlated systems, we study this pseudogapped incoherent metallic state in FeSe$_{1}$ − $_{x}$Te$_{x}$ in detail. S ( T, x ) data [9] reveal an approximate isosbectic point at T ≃ 100 K. This is strong evidence for a crossover scale, associated with effectively localized (Heikes law in S ( T )) to incoherent metallic conduction, in FeSe$_{1}$ − $_{x}$Te$_{x}$ . arXiv:1001.3273v1 [cond-mat.supr-con] 19 Jan 2010