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 pressure. We also observe a pressure induced orbital selective Lifshitz transition in ThFeAsN compound which is quite different from the Lifshitz transitions observed in the other families of Fe-based superconductors. Fermi surfaces of ThFeAsN specially hole like Fermi surfaces at $Γ$ point are altered significantly with pressure. This modification of Fermi surface topology with pressure seems to play the major role in the reduction of T$_c$ with pressure in ThFeAsN superconductor. Spin-orbit coupling does not affect the Lifshitz transition but it modifies the energy ordering of bands near $Γ$ point at higher pressure.
The recently synthesized ThFeAsN iron-pnictide superconductor exhibits a $T_c$ of 30 K, the highest of the 1111-type series in absence of chemical doping. To understand how pressure affects its electronic properties, we carried out microscopic investigations up to 3 GPa via magnetization, nuclear magnetic resonance, and muon-spin rotation experiments. The temperature dependence of the ${}^{75}$As Knight shift, the spin-lattice relaxation rates, and the magnetic penetration depth suggest a multi-band $s^{\pm}$-wave gap symmetry in the dirty limit, while the gap-to-$T_c$ ratio $Δ/k_\mathrm{B}T_c…
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…
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 b…
Among iron chalcogenide superconductors, FeS can be viewed as a simple, highly compressed relative of FeSe without nematic phase and with weaker electronic correlations. Under pressure, however, the superconductivity of stoichiometric FeS disappears and reappears, forming two domes. We perform electronic structure and spin fluctuation theory calculations for tetragonal FeS in order to analyze the nature of the superconducting order parameter. In the random phase approximation we find a gap function with d-wave symmetry at ambient pressure, in agreement with several reports of a nodal supercond…
The Fermi surface topology of $cI$16 Li at high pressures is studied using a recently developed first-principles unfolding method. We find the occurrence of a Lifshitz transition at $\sim$43 GPa, which explains the experimentally observed anomalous onset of the superconductivity enhancement toward lowered pressure. Furthermore we identify, in comparison with previous reports, additional nesting vectors that contribute to the $cI$16 structural stability. Our study highlights the importance of three-dimensional unfolding analyses for first-principles studies of Fermi surface topologies and insta…
Emerets's experiments on pressurized sulfur hydride have shown that H3S metal has the highest known superconducting critical temperature Tc=203K. The Emerets data show pressure induced changes of the isotope coefficient between 0.25 and 0.5, in disagreement with Eliashberg theory which predicts a nearly constant isotope coefficient. We assign the pressure dependent isotope coefficient to Lifshitz transitions induced by pressure and zero point lattice fluctuations. It is known that pressure could induce changes of the topology of the Fermi surface, called Lifshitz transitions, but were neglecte…
Motivated by recent experiments, we investigate the pressure-dependent electronic structure and electron-phonon (\emph{e-ph}) coupling for simple cubic phosphorus by performing first-principle calculations within the full potential linearized augmented plane wave method. As a function of increasing pressure, our calculations show a valley feature in T$_c$, followed by an eventual decrease for higher pressures. We demonstrate that this T$_c$ valley at low pressures is due to two nearby Lifshitz transitions, as we analyze the band-resolved contributions to the \emph{e-ph} coupling. Below the fir…
The maximum critical temperature for superconductivity in pressurized hydrides appears at the top of superconducting domes in Tc versus pressure curves at a particular pressure, which is not predicted by standard superconductivity theories. The a high-order anisotropic van Hove singularity near the Fermi level observed in band structure calculations of pressurized sulfur hydride, typical of a supermetal, has been associated with the array of metallic hydrogen wires modules forming a nanoscale heterostructure at atomic limit called superstripes phase. Here we propose that pressurized sulfur hyd…
Analogous to 2D layered transition metal dichalcogenides, the TlSe family of 1D chain materials with Zintl-type structure exhibits exotic phenomena under high-pressure. In the present work, we have systematically investigated the high-pressure behavior of TlInTe 2 using Raman spectroscopy, synchrotron X-ray diffraction, and transport measurements, in combination with crystal structure prediction (CSP) based on the evolutionary approach and first principles calculations. We found that TlInTe$_2$ undergoes a pressure driven semiconductor to semimetal transition at 4 GPa, followed by a supercondu…
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…
While 203 K high temperature superconductivity in H3S has been interpreted by BCS theory in the dirty limit here we focus on the effects of hydrogen zero-point-motion and the multiband electronic structure relevant for multigap superconductivity near Lifshitz transitions. We describe how the topology of the Fermi surfaces evolves with pressure giving different Lifshitz-transitions. A neck-disrupting Lifshitz-transition (type 2) occurs where the van Hove singularity, vHs, crosses the chemical potential at 210 GPa and new small 2D Fermi surface portions appear with slow Fermi velocity where the …
Pressure-induced Lifshitz and quantum phase transitions in electron-doped cuprate superconductor
Center for High Pressure Science & Technology Advanced Research（高压科学前沿中心）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；Shanghai Key Laboratory of Material Frontiers Research in Extreme Environments, Shanghai Advanced Research in Physical Sciences（极端环境材料前沿研究上海市重点实验室，上海物理科学研究院）；Maryland Quantum Materials Center, Department of Physics, University of Maryland（马里兰大学量子材料中心，物理学系）
The Fermi surface can be changed by different external conditions like, e.g., pressure or doping. It can lead to a change in the Fermi surface topology, called as the Lifshitz transition. Here, we briefly describe the Lifshitz transitions induced by the external magnetic field in a one-dimensional optical lattice and iron-based superconductors. We also discuss physical consequences emerging from these transitions.
An iron-based superconductor LaFeAsO$_{1-x}$H$_x$ (0 $\leq x \leq$ 0.6) undergoes two antiferromagnetic (AF) phases upon H doping. We investigated the second AF phase ($x$=0.6) using NMR techniques under pressure. At pressures up to 2 GPa, the ground state is a spin-density-wave state with a large gap; however, the gap closes at 4.0 GPa, suggesting a pressure-induced quantum critical point. Interestingly, the gapped excitation coexists with gapless magnetic fluctuations at pressures between 2 and 4 GPa. This coexistence is attributable to the lift up of the $d_{xy}$ orbital to the Fermi level,…
Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr$_{2}$RuO$_{4}$ for example, leading to a pronounced peak in $T_\mathrm{c}$ vs. strain whose origin is still under active debate. Here, we develop a simple and compact method to apply large uniaxial pressures passively in restricted sample environment…
By partially doping Pb to effectively suppress the superstructure in single-layered cuprate Bi2Sr2CuO6+δ(Pb-Bi2201) and annealing them in vacuum or in high pressure oxygen atmosphere, a series of high quality Pb-Bi2201 single crystals are obtained with Tc covering from 17 K to non-supercondcuting in the overdoped region. High resolution angle resolved photoemission spectroscopy (ARPES) measurements are carried out on these samples to investigate the evolution of the Fermi surface topology with doping in the normal state. Clear and complete Fermi surface are observed and quantitatively analyzed…
We present the evolution of the initial (up to ~ 10 kbar) hydrostatic, pressure dependencies of Tc and of the ambient pressure jump in the heat capacity associated with the superconducting transition as a function of K - doping in the Ba1-xKxFe2As2 family of iron-based superconductors. The pressure derivatives show weak but distinct anomaly near x ~ 0.7. In the same concentration region Delta Cp at Tc deviates from the Delta Cp ~ T^3 scaling found for most BaFe2As2 - based superconductors. These results are consistent with a Lifshitz transition, and possible significant modification of the sup…
We report measurements of the superconducting critical temperature Tc of polycrystalline MgB2 samples containing isotopically pure (10)B and (11)B under quasi-hydrostatic pressure conditions in He pressure media up to 44 GPa. Measurements to volume compressions V/V_0 ~ 0.82 allow us to observe a kink in the volume dependence of Tc for Mg(10)B2 (at 20 GPa) and Mg(11)B2 (at 15 GPa). The pressure dependence of the E(2g) mode also changes abruptly around 20 GPa for the Mg(10)B2 sample. The anharmonic character of the E(2g) phonon mode and anomalies in Tc pressure dependence are interpreted as the …
FeSe is a unique high-$T_c$ iron-based superconductor in which nematicity, superconductivity, and magnetism are entangled with each other in the $P$-$T$ phase diagram. We performed $^{77}$Se-nuclear magnetic resonance measurements under pressures of up to 3.9 GPa on 12% S-substituted FeSe, in which the complex overlap between the nematicity and magnetism are resolved. A pressure-induced Lifshitz transition was observed at 1.0 GPa as an anomaly of the density of states and as double superconducting (SC) domes accompanied by different types of antiferromagnetic (AF) fluctuations. The low-$T_{\rm…
Quasi-two-dimensional kagome metals AV3Sb5 (A = K, Rb, and Cs) have attracted much recent interest due to exotic quantum phenomena such as unconventional superconductivity, topological charge order and giant anomalous Hall effect. Here we report pressure-induced reemergent superconductivity in CsV3Sb5 by electrical transport measurements under high pressures up to 47.9 GPa. We show that the superconducting critical temperature Tc is first enhanced by pressure and reaches its first maximum ~ 8.9 K at 0.8 GPa, then the Tc is suppressed by pressure and cannot be detected above 7.5 GPa, forming a …
The transition metal dichalcogenide PdTe$_2$ has attractive features based on its classification as a type-II Dirac semimetal and the occurrence of type-I superconductivity, providing a platform for discussion of a topological superconductor. Our recent work revealed that type-I superconductivity persists up to pressures of $\sim2.5$ GPa and the superconducting transition temperature $T_{\rm c}$ reaches a maximum at around 1 GPa, which is inconsistent with the theoretical prediction. To understand its non-monotonic variation and investigate superconductivity at higher pressures, we performed s…
Here we report the discovery of pressure-induced superconductivity in quasi-one-dimensional $\mathrm{Ta}_2 \mathrm{PdSe}_6$, through a combination of electrical transport, synchrotron x-ray diffraction, and theoretical calculations. Our transport measurements show that the superconductivity appears at a critical pressure $P_{\mathrm{c}} \sim 18.3$ GPa and is robust upon further compression up to $62.6$ GPa. The estimated upper critical field $μ_0 H_{\mathrm{c} 2}(0)$ in the pressurized $\mathrm{Ta}_2 \mathrm{PdSe}_6$ is much lower than the Pauli limiting field, in contrast to the case in its i…
While it is known that a resonant amplification of Tc in two-gap superconductors can be driven by using the Fano-Feshbach resonance tuning the chemical potential near a Lifshitz transition, little is known on tuning the Tc resonance by cooperative interplay of the Rashba spin-orbit coupling (RSOC) joint with phonon mediated (e-ph) pairing at selected k-space spots. Here we present first-principles quantum calculation of superconductivity in an artificial heterostructure of metallic quantum wells with 3 nm period where quantum size effects give two-gap superconductivity with RSOC controlled by …
Investigating the pressure dependence of the superconducting (SC) transition temperature $T_{\rm c}$ is crucial for understanding the SC mechanism. Herein, we report on the pressure dependence of $T_{\rm c}$ in the nonmagnetic topological line-nodal material CaSb$_2$, based on measurements of electric resistance and alternating current magnetic susceptibility. $T_{\rm c}$ initially increases with increasing pressure and peaks at $\sim$ 3.1~GPa. With a further increase in pressure, $T_{\rm c}$ decreases and finally becomes undetectable at 5.9~GPa. Because no signs of phase transition or Lifshit…
The influence of carrier type on superconductivity has been an important issue for understanding both conventional and unconventional superconductors [1-7]. For elements that superconduct, it is known that hole-carriers govern the superconductivity for transition and main group metals [8-10]. The role of hole-carriers in elements that are not normally conducting but can be converted to superconductors, however, remains unclear due to the lack of experimental data. Here we report the first in-situ high pressure Hall effect measurements on single crystal black phosphorus, measured up to ~ 50 GPa…
Measurements of temperature-dependent resistance and magnetization under hydrostatic pressures up to 2.13 GPa are reported for single-crystalline, superconducting BaBi$_3$. A temperature - pressure phase diagram is determined and the results suggest three different superconducting phases $α$, $β$, and $γ$ in the studied pressure range. We further show that occurrence of the three superconducting phases is intuitively linked to phase transitions at higher temperature which are likely first order in nature. $T_p$, which separates phase $α$ from $β$ and $γ$, is associated with an abrupt resistanc…
A first-order-like resistivity hysteresis is induced by a subtle structural transition under hydrostatic pressure in the topological nodal-line superconductor PbTaSe$_2$. This structure transition is quickly suppressed to zero at pressure $\sim$0.25 GPa. As a result, superconductivity shows a marked suppression, accompanied with fundamental changes in the magnetoresistance and Hall resistivity, suggesting a Lifshitz transition around $\sim$0.25 GPa. The first principles calculations show that the spin-orbit interactions partially gap out the Dirac nodal line around $K$ point in the Brillouin z…
We report muon spin rotation experiments on the magnetic penetration depth lambda and the temperature dependence of lambda^{-2} in the over-doped Fe-based high-temperature superconductor (Fe-HTS) Ba_{1-x}Rb_ xFe_2As_2 (x = 0.65) studied at ambient and under hydrostatic pressures up to p = 2.3 GPa. We find that in this system lambda^{-2}(T) is best described by d-wave scenario. This is in contrast to the case of the optimally doped x = 0.35 system which is known to be a nodeless s^{+-}-wave superconductor. This suggests that the doping induces the change of the pairing symmetry from s^{+-} to d…
Effects of pressure on the electronic structure, electron-phonon interaction, and superconductivity of the high entropy alloy (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ are studied in the pressure range 0 - 100 GPa. The electronic structure is calculated using the Korringa-Kohn-Rostoker method with the coherent potential approximation. Effects of pressure on the lattice dynamics are simulated using the Debye-Grüneisen model and the Grüneisen parameter at ambient conditions. In addition, the Debye temperature and Sommerfeld electronic heat capacity coefficient were experimentally determined. The electron…
We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at the zero temperature limit, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and investigate what parameters control this transition. Comparing samples of different residual resistivity, we find …
Giant critical current peak induced by pressure in kagome superconductor RbV$_{3}$Sb$_{5}$
The Chinese University of Hong Kong（香港中文大学）；Southern University of Science and Technology（南方科技大学）；Victoria University of Wellington（惠灵顿维多利亚大学）；Nagoya University（名古屋大学）；Kyoto University（京都大学）
Emergence of Double-Dome Superconductivity in the Pressurized Dirac Semimetal BaMg2Bi2
State Key Laboratory of Quantum Functional Materials, School of Physical Science and Technology, ShanghaiTech University（上海科技大学物理科学与技术学院量子功能材料国家重点实验室）；ShanghaiTech Laboratory for Topological Physics, ShanghaiTech University（上海科技大学拓扑物理实验室）；Anhui Provincial Key Laboratory of Magnetic Functional Materials and Devices, Institutes of Physical Science and Information Technology, Anhui University（安徽大学物质科学与信息技术研究院安徽省磁性功能材料与器件重点实验室）；Shanghai Key Laboratory of High-resolution Electron Microscopy, ShanghaiTech University（上海科技大学高分辨电子显微镜上海市重点实验室）
We study the effect of hole-doping and chemical pressure (isovalent doping) in single crystals of K$_x$Eu$_{1-x}$Fe$_2$As$_2$ and EuFe$_2$(As$_{1-y}$P$_y$)$_2$, respectively, by measurements of the thermopower, $S(T)$, and electrical resistivity, $ρ(T)$. The evolution of $S(T)$ upon doping indicates drastic changes of the electronic configuration at critical values $x_{\mathrm{cr}}=0.3$ and $y_{\mathrm{cr}}=0.21$, respectively, as the spin-density-wave transition is completely suppressed and superconductivity (SC) emerges. For the case of chemical pressure, the comparison with published ARPES …
The topology of the Fermi surface of an extended $d-p$ Hubbard model is investigated using the Green's function technique in a n-pole approximation. The effects of the $d-p$ hybridization on the Fermi surface are the main focus in the present work. Nevertheless, the effects of doping, Coulomb interaction and hopping to second-nearest-neighbors on the Fermi surface, are also studied. Particularly, it is shown that the crossover from hole-like to electron-like Fermi surface (Lifshitz transition) is deeply affected by the $d-p$ hybridization. Moreover, the pseudogap present in the low doping regi…
Topological nodal-line semimetals (TNLSMs) are materials whose conduction and valence bands cross each other, meeting a topologically-protected closed loop rather than discrete points in the Brillouin zone (BZ). The anticipated properties for TNLSMs include drumhead-like nearly flat surface states, unique Landau energy levels, special collective modes, long-range Coulomb interactions, or the possibility of realizing high-temperature superconductivity. Recently, SrAs3 has been theoretically proposed and then experimentally confirmed to be a TNLSM. Here, we report high-pressure experiments on Sr…
The nematic electronic state and its associated nematic critical fluctuations have emerged as potential candidates for superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with other magnetically-ordered phases poses significant challenges in establishing their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe$_{0.89}$S$_{0.11}$, we provide a unique access to a clean nematic quantum phase transition in the absence of a long-range magnetic order. We find that in the proximity of the nematic phase transiti…
We investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition in single crystals of FeSe1-xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 tesla in the low temperature regime. The unusually small and strongly elongated Fermi surface of FeSe increases monotonically with chemical pressure, x, due to the suppression of the in-plane anisotropy except for the smallest orbit which suffers a Lifshitz-like transition once nematicity disappears. Even outside the nematic phase the Fermi surface continues to increase, in stark …
