Pressure induced Lifshitz transition and anomalous crystal field splitting in AFeAs (A=Li/Na) Fe-based superconducting compounds: A first principles study
The effect of hydrostatic pressure on the electronic structures of iron pnictide superconducting compounds LiFeAs and NaFeAs through density functional theory based first principles studies are presented, using the predicted crystal structures at very high pressure by Zhang et al. [1]. The orbital selective pressure induced modifications in the partial density of states and electronic band structures reveal mixed multi-band multi-orbital nature of these compounds, with energetically degenerate dxz/dyz orbitals at ambient pressure. Due to larger hydrostatic pressures some of the electron/hole bands crosses the Fermi level, leading to significant topological modifications in Fermi surfaces known as Lifshitz transition. Interrelation between Lifshitz transition and superconductivity-an important subject matter of current research are described. Based on such interconnection the current study predicts the superconducting-Tc in 111 compounds can not be raised at such high pressures. Wannier functions based electronic structure investigation reveal a relatively larger hybridization between the Fe-3d and As-4p orbitals at higher pressures, as an effect of which the orbital degeneracy of dxz and dyz orbitals are lifted. Different hybridization contributions in the crystal-field splitting are separated using the Wannier functions based formalism, by incorporating different bands with a particular orbital character in the Wannier function construction. Pressure dependence of the intra/inter orbital hopping amplitudes between Fe-d orbitals has been discussed using low energy tight binding model.
A review of high-pressure studies on Fe-pnictide superconductors is given. The pressure effects on the magnetic and superconducting transitions are discussed for different classes of doped and undoped FeAs-compounds, ROFeAs (R = rare earth), AeFe2As2 (Ae = Ca, Sr, Ba), and AFeAs (A = Li, Na). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting Tc increases with pressure for underdoped FeAs-pnictides, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. The undoped LaOFeA…
In this paper, we present pressure dependent structural parameters and electronic structure of ThFeAsN superconductor. There are no anomalies in the structural parameters as well as elastic constants with hydrostatic pressure which is consistent with the experiments. We study the electronic structure of this compound at different external pressures in terms of density of states, band structure and Fermi surface. Density of states at the Fermi level, coming from Fe-d orbitals follows the same trend as that of the superconducting transition temperature (T$_c$) as a function of hydrostatic pressu…
A comprehensive first principles study on the electronic topological transition in a number of 122 family of Fe based superconductors is presented. Doping as well as temperature driven Lifshitz transitions are found from first principles simulations in a variety of Fe based superconductors that are consistent with experimental findings. In all the studied compounds the Lifshitz transitions are consistently found to occur at a doping concentration where superconductivity is highest and magnetism disappears. Systematically, the Lifshitz transition occurs in the electron Fermi surfaces for hole d…
We demonstrate, using $T=0$ first principles total energy calculations, that observed pressure-driven anomalies in the entire 122-pnictides family ($A$Fe$_2$As$_2$; $A$ = alkali earth element Ca, Sr, Ba) can be understood as consequences of Lifshitz transitions (LTs) \cite{Lifshitz60}. Our results for energy band dispersions and spectra, lattice parameters, enthalpies, magnetism, and elastic constants over a wide range of hydrostatic pressure provide a coherent understanding of multiple transitions in these compounds, namely, enthalpic, magnetic and tetragonal (T) - collapsed tetragonal (cT) t…
A combination of phenomenological analysis and Mössbauer spectroscopy experiments on the tetragonal Fe$_{1+y}$Te system indicates that the magnetic ordering transition in compounds with higher Fe-excess, $y\ge$ 0.11, is unconventional. Experimentally, a liquid-like magnetic precursor with quasi-static spin-order is found from significantly broadened Mössbauer spectra at temperatures above the antiferromagnetic transition. The incommensurate spin-density wave (SDW) order in Fe$_{1+y}$Te is described by a magnetic free energy that violates the weak Lifshitz condition in the Landau theory of seco…
The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, has been a longstanding puzzle. Here we consider the effect of impurity scattering on the temperature dependence of the average resistivity within a simple two-band model of a dirty spin density wave metal. The sharp drop in resistivity below the Néel temperature $T_N$ in the parent compound can only be understood in terms of a Lifshitz transition following Fermi surface reconstruction upon magnetic ordering. We show …
According to the celebrated Onsagar-Lifshitz paradigm, the observation of Shubnikov de-Haas and de-Haas van Alphen (SdHvA) oscillations is an indication of the presence of `closed' orbit Fermi surface in the bulk. We present a real-space based calculation of SdHvA oscillations in generalized quasi-one-dimensional lattices by relaxing the quasi-classical approximations embedded in this decades old Onsagar-Lifshitz paradigm. We find that sizable quantum oscillation can arise from `open' Fermi surfaces as long as cyclotron orbits can form in real-space with finite, but not necessarily equal, elec…
How the macroscopic quantum coherence can resist to the decoherence attacks of high temperature is a major challenge for the science of the 21st century. Superstripes 2017 conference held in Ischia on June 2017 has been focused on the new physics of high $T_c$ superconductors made of complex quantum matter. Today the standard model of high $T_c$ superconductivity which grabs the physics of complex quantum matter is the multi-band Hubbard model where the dome of $T_c$ occurs by driving the chemical potential in the proximity of a topological Lifshitz transition. The multi-gap superconductivity …
According to the Bardeen-Cooper-Schrieffer (BCS) theory, superconductivity usually needs well defined Fermi surface(s) with strong electron-phonon coupling and moderate quasiparticle density of states (DOS). A kagome lattice can host flat bands and topological Dirac bands; meanwhile, due to the parallel Fermi surfaces and the saddle points, many interesting orders are expected. Here, we report the observation of superconductivity by pressurizing a kagome compound Rb$_2$Pd$_3$Se$_4$ using a DAC anvil. The parent compound shows an insulating behavior; however, it gradually becomes metallic and t…
In this paper we address Lifshitz transition induced by applied external magnetic field in a case of iron-based superconductors, in which a difference between the Fermi level and the edges of the bands is relatively small. We introduce and investigate a two-band model with intra-band pairing in the relevant parameters regime to address a generic behaviour of a system with hole-like and electron-like bands in external magnetic field. Our results show that two Lifshitz transitions can develop in analysed systems and the first one occurs in the superconducting phase and takes place at approximate…
We show that only a few percentage of Sn doping at the Ba site on BaFe$_2$As$_2$, can cause electronic topological transition, namely, the Lifshitz transition. A hole like d$_{xy}$ band of Fe undergoes electron like transition due to 4\% Sn doping. Lifshitz transition is found in BaFe$_2$As$_2$ system around all the high symmetry points. Our detailed first principles simulation predicts absence of any Lifshitz transition in other 122 family compounds like SrFe$_2$As$_2$, CaFe$_2$As$_2$. This work bears practical significance due to the facts that a few percentage of Sn impurity is in-built in …
Herewith, we review the available experimental data of thermoelectric transport properties of iron-based superconductors and parent compounds. We discuss possible physical mechanisms into play in determining the Seebeck effect, from whence one can extract information about Fermi surface reconstruction and Lifshitz transitions, multiband character, coupling of charge carriers with spin excitations and its relevance in the unconventional superconducting pairing mechanism, nematicity, quantum critical fluctuations close to the optimal doping for superconductivity, correlation. Additional informat…
We review our recent studies on ferromagnetic superconductors, UGe2, URhGe and UCoGe, together with the ferromagnetic quantum criticality and paramagnetic singularity on the Ising 5f-itinerant system UCoAl. Thanks to the variety of ordered moment in ferromagnetic superconductors from 1.5 muB to 0.05 muB, interesting systematic changes or similarities are clarified. All ferromagnetic superconductors show large upper critical field Hc2, and the field-reentrant (-reinforced) phenomena are observed in the field-temperature phase diagram, when the pressure or field direction is tuned for particular…
Using the experimentally measured temperature and doping dependent structural parameters on Ru doped BaFe$_2$As$_2$, orbital-dependent reconstruction of the electronic structure across the magnetostructural transition is found, through first principle simulations. Below structural transition there exists two distinct Fe - Fe bond distances which modifies the Fe-d$_{xy}$ orbital largely due to its planar spatial extension leading to Lifshitz transition, while the otherwise degenerate Fe-d$_{xz}$ and d$_{yz}$ orbitals become non-degenerate, giving rise to orbital order. The orbital order follows…
The spin dynamics in single crystal, electron-doped Ba(Fe1-xCox)2As2 has been investigated by inelastic neutron scattering over the full range from undoped to the overdoped regime. We observe damped magnetic fluctuations in the normal state of the optimally doped compound (x=0.06) that share a remarkable similarity with those in the paramagnetic state of the parent compound (x=0). In the overdoped superconducting compound (x=0.14), magnetic excitations show a gap-like behavior, possibly related to a topological change in the hole Fermi surface (Lifshitz transition), while the imaginary part of…
Nematicity in the heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from a magnetic instability were declared in RbFe$_2$As$_2$ and its sister compounds. Here we report a systematic elastoresistance study of series of isovalent- and electron-doped KFe$_2$As$_2$ crystals. We found divergent elastoresistance upon cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven towards a Lifshitz transition. However, w…
We construct a phase diagram of the parent compound Fe1+xTe as a function of interstitial iron x in terms of the electronic, structural, and magnetic properties. For a concentration of x < 10%, Fe1+xTe undergoes a "semimetal" to metal transition at approximately 70 K that is also first-order and coincident with a structural transition from a tetragonal to a monoclinic unit cell. For x ~ 14%, Fe1+xTe undergoes a second-order phase transition at approximately 58 K corresponding to a "semimetal" to "semimetal" transition along with a structural orthorhombic distortion. At a critical concentration…
Hund's coupling assisted orbital-selective superconductivity in Ba1-xKxFe2As2
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料能源科学研究所，SLAC国家加速器实验室）；Geballe Laboratory for Advanced Materials, Department of Applied Physics and Physics, Stanford University（斯坦福大学应用物理与物理系，Geballe先进材料实验室）；Department of Applied Physics, Stanford University（斯坦福大学应用物理系）；Department of Applied Physics, KTH-Royal Institute of Technology（瑞典皇家理工学院应用物理系）；National Institute of Advanced Industrial Science and Technology (AIST)（日本产业技术综合研究所）；Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory（SLAC国家加速器实验室斯坦福同步辐射光源）；Department of Material Science, Stanford University（斯坦福大学材料科学系）
We analyze the two-orbital (with $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals) analogue of the $t$-$J$-$U$ model as applied to the description of the copper-oxide monolayer deposited on the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ substrate (CuO$_2$/BSCCO). We show, that an $extended$ $s$-$wave$ superconducting dome appears in the heavily hole overdoped regime of the model, with dominant contribution to the pairing coming from the $d_{3z^2-r^2}$ orbitals. Also, the lower critical doping for the appearance of the SC state concurs with the Lifshitz Transition after which the hole-like Fermi pockets are create…
Recently discovered heavy fermion CeRh$_{2}$As$_{2}$ compound crystallizes in the nonsymmorphic $P4/nmm$ symmetry, which enables the occurrence of topological protection. Experimental results show that this material exhibits unusual behavior, which is manifested by the appearance of two superconducting phases. In this work, we uncover and discuss a role of Rh$_{2}$As$_{2}$ layers and their impact on the electronic and dynamical properties of the system. The location of Ce atoms between two non-equivalent layers allows for the realization of hidden orbital order. We point out that the electroni…
A rich variety of nematic/smectic orders in Fe-based superconductors is an important unsolved problem in strongly correlated electron systems. A unified understanding of these orders has been investigated for the last decade. In this article, we explain the $B_{1g}$ symmetry nematic transition in FeSe$_{1-x}$Te$_x$, the $B_{2g}$ symmetry nematicity in AFe$_2$As$_2$ (A=Cs, Rb), and the smectic state in BaFe$_2$As$_2$ based on the same framework. We investigate the quantum interference mechanism between spin fluctuations by developing the density wave equation. The observed rich variety of nemat…
