A simple empirical relation has been found to exist between optimum Tc and the formal mean cation charge <qc> of perovskite-related superconductors, covering both conventional superconductors and superconducting cuprates. Tc is shown to increase exponentially with decreasing <qc>. It is suggested that a Ba-based cuprate with <qc> about 2 reach a Tc around 200 K. The strong correlation may be thought of as an indication for a common mechanism of superconductivity of the whole family of compounds. In addition, the structural assignment for the 'cubic' high-Tc phase reported by Volkov (Tc = 117 K) to the orthorhombic BaCuO2.5 prototype is proposed.
We have synthesized a new superconductor ZnNyNi3 with Tc ~3 K. The crystal structure has the same anti-perovskite-type such as MgCNi3 and CdCNi3. As far as we know, this is the third superconducting material in Ni-based anti-perovskite series. For this material, superconducting parameters, lower-critical field Hc1(0), upper-critical field Hc2(0), coherence length x(0), penetration depth l(0), and Gintzburg -Landau parameter k(0) have been experimentally determined.
We report the pressure dependence of the structural parameters of the non-oxide perovskite superconductor MgCNi$_{3}$ up to 32 GPa using a diamond anvil cell and synchrotron x-rays at room temperature. The structure of the compound remains in the Pm-3m cubic symmetry throughout the pressure range. The bulk modulus $B_{0}=156.9\pm0.2$GPa with $B_{0}^{\prime}=9.8$obtained by fitting the pressure-volume data is in good agreement with theoretical calculations reported earlier. An anomalous shift of the (111) and (200) lines observed above 9 GPa indicates a possible local short range distortion tha…
We report the structural parameters of superconducting MgCxNi3 (x=0.96, TC=7.3 K) as a function of temperature, from 2 K to 295 K, determined by neutron powder diffraction profile refinement. The compound has the perovskite structure over the whole temperature range, with symmetry Pm3m and a=3.81221(5) A at 295 K: no structural or long range magnetic ordering transitions were observed. The lattice parameter a and the Debye-Waller factors for the individual atoms decrease smoothly with decreasing temperature. There are no unusual changes of the structural parameters near TC.
Full-potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential has been applied for the study of structural, elastic and electronic properties of the newly synthesized nitrogen-containing perovskite-like superconductor ZnNNi3. The optimized lattice parameter, independent elastic constants (C11, C12 and C44), bulk modulus B, compressibility betta, and shear modulus G are evaluated. The band structure, total and site- projected l- decomposed DOSs, the shape of the Fermi surface, the Sommerfeld coefficient a…
Local and fast structural probes using synchrotron radiation have shown nanoscale striped puddles and nanoscale phase separation in doped perovskites.It is known that the striped phases in doped perovskites are due to competing interactions involving charge, spin and lattice degrees of freedom,but while many theoretical models for spin and charge stripes have been proposed we are missing theoretical models describing the complex lattice striped modulations in doped perovskites. In this work we show that two different stripes can be represented as a superposition of a pair of stripes, U(theta_n…
A pivotal challenge posed by unconventional superconductors is to unravel how superconductivity emerges upon cooling from the generally complex normal state. Some of the most prominent unconventional superconductors are oxides: strontium titanate, strontium ruthenate, and the cuprates exhibit greatly different superconducting transition temperatures $T_c$, and although their respective superconducting pairing mechanisms remain unknown, they are thought to differ as well. Here we use nonlinear magnetic response, a probe that is uniquely sensitive to the superconducting precursor, to uncover rem…
We have measured upper-critical-field $H_{\text c2}$, specific heat C, and tunneling spectra of the intermetallic perovskite superconductor MgCNi${}_3$ with a superconducting transition temperature $T_{\text c}\approx 7.6$ K. Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field $H_{\text c}$(0), coherence length $ξ$(0), penetration depth $λ$(0), lower-critical-field $H_{\text c1}$(0), and Ginsberg-Landau parameter $κ$(0). From the specific heat, we obtain the Debye temp…
We have measured the cyclotron masses in Sr2RuO4 through the observation of periodic-orbit-resonances - a magnetic resonance technique closely related to cyclotron resonance. We obtain values for the alpha, beta and gamma Fermi surfaces of (4.33+/-0.05)me, (5.81+/-0.03)me and (9.71+/-0.11)me respectively. The appreciable differences between these results and those obtained from de Haas- van Alphen measurements are attributable to strong electron-electron interactions in this system. Our findings appear to be consistent with predictions for a strongly interacting Fermi liquid; indeed, semi-quan…
In a recent paper (Phys. Rev. {\bf B 67}, 094502 (2003)) Mao et al. investigated the bias-dependent conductance of mechanical junctions between superconducting MgCNi$_3$ and a sharp W tip. They interpreted their results in terms of 'single-particle tunneling'. We show it is more likely that current transport through those junctions is determined by thermal effects due to the huge normal-state resistivity of MgCNi$_3$. Therefore no conclusion can be drawn about the possible unconventional pairing or strong-coupling superconductivity in MgCNi$_3$.
Inverse Isotope Effect in the Ternary Perovskite Hydride SrPdH/D$_{2.9}$: A Signature of Quantum Zero-Point Fluctuations
Carnegie Institution for Science（卡内基科学研究所）；Graz University of Technology（格拉茨工业大学）；Stockholm University（斯德哥尔摩大学）；Intellectual Ventures（智识风险投资公司）；University of Cambridge（剑桥大学）；Tohoku University（东北大学）；Oak Ridge National Laboratory（橡树岭国家实验室）
Raman spectroscopy is a very powerful probe to study the nature of quasi-particle excitations in condensed matter physics. The work presented in this thesis is focused on two different families of novel materials, namely the iron-based superconductors (FeBS), multiferroic oxides and double perovskite. Although the properties of these two systems are quite different, some comparison can still be drawn between them. For instance, in both of these systems magnetism plays a crucial role and intricate coupling between phononic, magnetic and orbital degrees of freedom is crucial to understand their …
A possibility for a new family of intermetallic perovskite superconductors Sc3XB, with X = Tl, In, Ga and Al, is presented as a result of KKR electronic structure and pseudopotential phonon calculations. The large values of computed McMillan--Hopfield parameters on scandium suggest appearance of superconductivity in Sc3XB compounds. On the other hand, the possibility of weak itinerant ferromagnetic behavior of Sc3X systems is indicated by the small magnetic moment on Sc atoms in two cases of X =~ l and In. Also the electronic structure and resulting superconducting parameters for more realisti…
Here we propose a possible way to produce superconductors with high critical temperature Tc by confinement of a Fermi liquid in a superlattice of quantum stripes (wells, wires or dots). The enhancement of Tc is obtained by tuning the size L of the stripes (wells, wires or dots), and the wavelength of electrons at the Fermi level λF at the resonance condition L λF. The 3D superconducting phase is stabilized by the condition that the separation W between the stripes (wells, wires or dots) should be of the order of the coherence length ξ0. This quantum state is realized in Bi2Sr2Ca2Cu2O8+y superc…
$\mathrm{YBa_2Cu_3O_7/La_{2/3}Ca_{1/3}MnO_3}$ superconducting/ferromagnetic (SC/FM) multilayers have been studied by neutron reflectometry. Evidence for a characteristic difference between the structural and magnetic depth profiles is obtained from the occurrence of a structurally forbidden Bragg peak in the FM state. The comparison with simulated reflectivity curves allows us to identify two possible magnetization profiles: a sizable magnetic moment within the SC layer antiparallel to the one in the FM layer (inverse proximity effect), or a ``dead'' region in the FM layer with zero net magnet…
In this present study, the pseudopotential plane-wave (PP-PW) pathway in the scheme of density functional theory (DFT) is utilized to investigate the various physical properties on (Ba0.82K0.18)(Bi0.53Pb0.47)O3 (BKBPO) single perovskite superconductor. We have analyzed elastic constants and moduli at zero and elevated pressures (up to 25 GPa) as well. We also have investigated the anisotropic nature incorporating both the theoretical indices and graphical representations in 2D and 3D dimensions, which reveals a high level of anisotropy. The flatness of the energy bands near EF is a sign of Van…
We report the pressure dependences of the superconducting transition temperature (T_c) in several perovskite-type Fe-based superconductors through the resistivity measurements up to ~4 GPa. In Ca_4(Mg,Ti)_3Fe_2As_2O_y with the highest T_c of 47 K in the present study, the T_c keeps almost constant up to ~1 GPa, and starts to decrease above it. From the comparison among several systems, we obtained a tendency that low T_c with the longer a-axis length at ambient pressure increases under pressure, but high T_c with the shorter a-axis length at ambient pressure hardly increases. We also report th…
We report a detailed study of the magnetic-field-orientation dependence of the millimetre-wave magnetoconductivity of the superconductor Sr2RuO4 We find two harmonic series of cyclotron resonances. We assign the first, corresponding to a quasiparticle mass of $4.29 \pm 0.05 m_{\rm e}$, where $m_{\rm e}$ is the free-electron mass, to the $β$ Fermi-surface section. We assign the second series, which contains only odd harmonics, to cyclotron resonance of the $γ$ Fermi-surface section, yielding a quasiparticle mass of $12.35 \pm 0.20 m_{\rm e}$. A third, single cyclotron resonance, corresponding t…
The recent discovery nickel oxides superconductors have highlighted the importance of first-principles simulations for understanding the formation of the bound electrons at the core of superconductivity. Nevertheless, superconductivity in oxides is often ascribed to strong electronic correlation effects that Density Functional Theory (DFT) cannot properly take into account, thereby disqualifying this technique. Being isostructural to nickel oxides, Sr1-xKxBiO3 superconductors form an ideal testbed for unveiling (i) the lowest theory level needed to model complex superconductors and (ii) the un…
Drawing the analogy to BaBiO3, we investigate via ab-initio electronic structure calculations potential new superconductors of the type ATlX3 with A = Rb, Cs and X = F, Cl, and Br, with a particular emphasis on RbTlCl3. Based on chemical reasoning, supported by the calculations, we show that Tl-based perovskites have structural and charge instabilities driven by the lone pair effect, similar to the case of BaBiO3, effectively becoming A2Tl1+Tl3+X6. We find that upon hole doping of RbTlCl3, structures without Tl1+, Tl3+ charge disproportionation become more stable, although the ideal cubic pero…
We have discovered a new homologous series of iron pnictide oxides (Fe2As2)(Can+2(Al,Ti)nOy)[n = 2,3,4]. These compounds have perovskite-like blocking layers between Fe2As2 layers. The structure of new compounds are tetragonal with space groups of P4/nmm for n = 2 and 4 and P4mm for n = 3, which are similar to those of (Fe2As2)(Can+1(Sc,Ti)nOy)[n = 3,4,5] found in our previous study. Compounds with n = 3 and 4 have new crystal structures with 3 and 4 sheets of perovskite layers, respectively, including a rock salt layer in each blocking layer. The a-axis lengths of the three compounds are appr…
The effects of spin-polarized quasiparticle transport in superconducting YBa2Cu3O_{7-δ} (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. These effects are compared with the injection of simple quasiparticles into control samples of perovskite non-magnetic metal-insulator-superconductor (N-I-S). Systematic studies of the critical current density as a function of the injection current density, temperature and the thickness of the superconductor reveal drastic differences between the F-I-S and N-I-…
Despite the discovery of new superconductors classes, high-Tc oxides continue to be a current topic, because of their complex phase diagrams and doping-dependant effects (allowing one to investigate the interaction between orbitals), as well as structural properties such as lattice distortion and charge ordering, among many others. Ruthenocuprates are magnetic superconductors in which the magnetic transition temperature is much higher than the critical superconducting temperature, making them unique compounds. With the aim of investigating the dilution of the magnetic Ru sub-lattice, we propos…
The tri-layered perovskite Sr4Rh3O10 is reported for the first time. High-pressure and high-temperature heating (6 GPa and 1500 C) brought about successful preparation of a polycrystalline sample of the expected member at n=3 of Srn+1RhnO3n+1. Neutron-diffraction studies revealed the orthorhombic crystal structure (Pbam) at room temperature and 3.4 K. Local structure distortions rotationally tilt the RhO6 octahedra ~12 degrees in the perovskite-based blocks along the c-axis, and approximately a 20 % disorder was found in sequence of the alternating rotational tilt. The sample was also investig…
Much of the current experimental efforts for detecting Majorana zero modes have been centered on probing the boundary of quantum wires with strong spin-orbit coupling. The same type of Majorana zero mode can also be realized at crystalline dislocations in 2D superconductors with the nontrivial weak topological indices. Unlike at an Abrikosov vortex, at such a dislocation, there are no other low-lying midgap states than the Majorana zero mode so that it avoids usual complications encountered in experimental detections such as scanning tunneling microscope (STM) measurements. We will show that, …
Mixed-valent perovskite oxides based on BaBiO3 (BBO) are, like cuperates, well-known high-Tc superconductors. Recent ab inito calculations have assigned the high-Tc superconductivity to a correlation-enhanced electron--phonon coupling mechanism, stimulating the prediction and synthesis of new superconductor candidates among mixed-valent thallium perovskites. Existing superconductivity has meant that research has mainly focused on hole-doped compounds, leaving electron-doped compounds relatively unexplored. Here we demonstrate through ab inito calculations that BBO emerges as a topological insu…
This review article is a commemoration of the 30th anniversary of the discovery of YBa2Cu3O7-delta high-temperature superconductors (HTSCs). As a result of this discovery a family of (RE)Ba2Cu3O7-delta (RE stands for "rare earth") HTSCs has found great practical use. The review article consists of a brief history of how YBa2Cu3O7-delta was conceived and five sections describing the family of compounds: crystallography, phase diagrams, manufacturing techniques, main superconducting properties, and fields of application.
Elastic neutron scattering simultaneously probes both the crystal structure and magnetic order in a material. Inelastic neutron scattering measures phonons and magnetic excitations. Here, we review the average composition, crystal structure and magnetic order in the 245 family of Fe-based superconductors and in related insulating compounds from neutron diffraction works. A three-dimensional phase-diagram summarizes various structural, magnetic and electronic properties as a function of the sample composition. A high pressure phase diagram for the superconductor is also provided. Magnetic excit…
The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero-field, time-reversal symmetry breaking has been observed in Re6Hf. Moreover, the temperature dependence of the superfluid density exhibits s-wave superconductivity with an enhanced electron-phonon coupling. This, coupled with the results from isostructural Re6Zr, shows that the Re6X family are indeed a new and important group of unconventional superconductors.
We describe three previously unreported superconductors, BaPb3, Ba0.89Sr0.11Pb3 and Ba0.5Sr0.5Pb3. These three materials, together with SrPb3, form a distinctive isoelectronic family of intermetallic superconductors based on the stacking of Pb planes, with crystal structures that display a hexagonal to cubic perovskite-like progression, as rarely seen in metals. The superconducting transition temperatures (Tc) are similar for all - 2.2 K for BaPb3, 2.7 K for Ba0.89Sr0.11Pb3 and 2.6 K for Ba0.5Sr0.5Pb3, and the previously reported Tc of SrPb3, ~ 2 K, is confirmed. The materials are moderate cou…
Fermiology of various 122 systems are studied through first principles simulation. Electron doping causes expansion of electron and shrinkage of hole Fermi pockets. Isovalent Ru substitution (upto 35%) makes no visible modification in the electron and hole like FSs providing no clue regarding the nature of charge carrier doping. However, in case of 32% P doping there are considerable changes in the hole Fermi surfaces (FSs). From our calculations, it is very clear that two dimensionality of FSs may favour electron pair scattering between quasi-nested FSs which has important bearings in various…
An index for the characterisation of the Perovskite type Superconductors is introduced. The index, denominated as Structural Parameter of High Tc Superconductivity (PESATc from its name in Spanish), is calculated using a Moebius Orbital defined as the length of an Octahedral Moebius Strip overlaying the octahedra created by the CuO planes and the apical Oxygen atoms, O(4). Results for structures of YBCO:123 type compounds are presented; their atomic positions were measured by neutron diffraction and x-ray diffraction techniques as reported in the literature. It has been found that critical tem…
We show using density functional calculations that quasi-one-dimensional K2Mo3As3, which is closely related to the K2Cr3As3 and has very similar superconducting properties, is not close to magnetism and has conventional s-wave electron-phonon superconductivity. This superconductivity is of multi-gap character due to different coupling on different Fermi surface sheets. This is discussed in relation to the properties of this family of quasi-one-dimensional pnictide superconductors. The results show that this family of superconductors provides a unique opportunity for studying the interplay of s…
Normal-state magnetic properties of cuprate high-T_c superconductors are interpreted based on the self-consistent solution of the t-J model of Cu-O planes. The solution method retains the rotation symmetry of spin components in the paramagnetic state and has no preset magnetic ordering. The obtained solution is homogeneous. The calculated temperature and concentration dependencies of the magnetic susceptibility are close to those observed in experiment. These results offer explanations for the observed scaling of the static uniform susceptibility and for the changes in the spin correlation len…
We refined crystal structures of newly found members of the Fe-1111 superconductor family, CaFe\_{1-x}Co\_{x}AsF and SrFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) by powder synchrotron X-ray diffraction analysis. The tetragonal to orthorhombic phase transitions were observed at ~120 K for unsubstituted CaFeAsF and at ~180 K for unsubstituted SrFeAsF, the transition temperatures agreeing with kinks observed in temperature-dependent resistivity curves. Although the transition temperature decreases, the structural phase transitions were observed below 100 K in both samples of x = 0.06, and finally th…
We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr$_{1-x}$Na$_{x}$Fe$_2$As$_2$. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels ($x \gtrsim 0.48$, near optimal superconductivity) and high temperatures (above room temperature for $x = 0$, decreasing…
We present a theoretical ab-initio approach that allows us to explicitly calculate the superconducting transition temperatures (Tc) of the iron-based superconductors of LaFeAsO1-xFx, SmFeAsO1-xFx, NdFeAsO1-xFx, Ba1-xKxFe2As2, FeSe and LiFeAs that fit perfectly with the experiments. We consider recent evidence that electron-phonon coupling may have been underestimated previously, and a prediction that antiferromagnetism can greatly enhance electron-phonon coupling through localized iron d orbitals. We then include the contribution of these localized orbitals in a McMillan formalism. In addition…
We predict that the square lattice layer formed by [Co$_2$N$_2$]$^{2-}$ diamond-like units can host high temperature superconductivity. The layer appears in the stable ternary cobalt nitride, BaCo$_2$N$_2$. The electronic physics of the material stems from Co$_2$N$_2$ layers where the dimerized Co pairs form a square lattice. The low energy physics near Fermi energy can be described by an effective two orbital model. Without considering interlayer couplings, the two orbitals are effectively decoupled. This electronic structure satisfies the "gene" character proposed for unconventional high tem…
The fluorine-doped rare-earth iron oxyarsenides, REFeAsO1-xFx (RE =rare earth) have recently emerged as a new family of high-temperature superconductors with transition temperatures (Tc) as high as 55 K (refs 1-4). Early work has provided compelling evidence that the undoped parent materials exhibit spin-density-wave (SDW) antiferromagnetic order and undergo a structural phase transition from tetragonal to orthorhombic crystal symmetry upon cooling.5 Both the magnetic and structural instabilities are suppressed upon doping with fluoride ions before the appearance of superconductivity.6,7 Here …
In addition to higher Tc compared with the ubiquitous cuprates for a material composed of a single electronically active layer, the newly discovered LnFeAsO superconductors offer additional compositional variation. In a similar fashion to the CuO2 layers in cuprates, the FeAs layers now dominate the electronic states that produce superconductivity. Cuprate superconductors distinguish themselves structurally by adopting different stacking of the Cu-O and electronically inactive "spacer" layers. Using the same structural philosophy, materials with the formula (A,K)Fe2As2,A=Ba or Sr have been rep…
We have investigated the superconducting state of the noncentrosymmetric superconductor Re6Ti (Tc = 6.0 K) using muon-spin rotation/relaxation (muSR) technique. The zero-field muon experiment shows the presence of spontaneous magnetic fields in the superconducting state, indicating time-reversal symmetry breaking (TRSB). However, the low-temperature transverse field muon measurements suggest nodeless s-wave superconductivity. The time reversal symmetry breaking further confirmed in the stoichiometric composition Re24Ti5. These results indicate that the pairing symmetry is not affected by spin-…
The advances in the field of unconventional superconductivity are largely driven by the discovery of novel superconducting systems. Here we report on the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional RbMn6Bi5 via measurements of resistivity and magnetic susceptibility under high pressures. With increasing pressure, its antiferromagnetic transition with TN = 83 K at ambient pressure is first enhanced moderately and then suppressed completely at a critical pressure of Pc = 13 GPa, around which bulk superconductivity emerges and exhibits a dome…
A newly discovered family of the Fe-based superconductors is isostructural with the so-called 122 family of Fe pnictides, but has a qualitatively different doping state. Early experiments indicate that superconductivity is nodeless, yet prerequisites for the $s_\pm$ nodeless state (generally believed to be realized in Fe superconductors) are missing. It is tempting to assign a $d-$ wave symmetry to the new materials, and it does seem at first glance that such a state may be nodeless. Yet a more careful analysis shows that it is not possible, given the particular 122 crystallography, and that t…
We report superconductivity and magnetism in a new family of topological semimetals, the ternary half Heusler compounds $R$PdBi ($R$ : rare earth). In this series, tuning of the rare earth $f$-electron component allows for simultaneous control of both lattice density via lanthanide contraction, as well as the strength of magnetic interaction via de Gennes scaling, allowing for a unique tuning of both the normal state band inversion strength, superconducting pairing and magnetically ordered ground states. Antiferromagnetism with ordering vector (0.5,0.5,0.5) occurs below a Neéel temperature tha…
We propose a simple way to stabilize half-quantum vortices in superconducting strontium ruthenate, assuming the order parameter is of chiral $p_x + ip_y$ symmetry, as is suggested by recent experiments. The method, first given by Salomaa and Volovik in the context of Helium-3, is very naturally suited for strontium ruthenate, which has a layered, quasi-two-dimensional, perovskite crystal structure. We propose possible experiments to detect their non abelian-braiding statistics. These experiments are of potential importance for topological quantum computation.
We investigated the pressure effects on the FeSe superconductor and the related compounds. Pressure dependence of superconducting transition temperature (Tc) for FeSe0.8S0.2 exhibits a dome-shaped behavior below 0.76 GPa. On the other hand, the Tc of FeSe0.25Te0.75 linearly increases up to 0.99 GPa. Here we discuss the relation between the physical pressure effects and the chemical pressure effects on the FeSe system.
We suggest that a family of Ni-based compounds, which contain [Ni$_2$M$_2$O]$^{2-}$(M=chalcogen) layers with an antiperovskite structure constructed by mixed-anion Ni complexes, NiM$_4$O$_2$, can be potential high temperature superconductors upon doping or applying pressure. The layer structures have been formed in many other transitional metal compounds such as La$_2$B$_2$Se$_2$O$_3$(B=Mn, Fe,Co). For the Ni-based compounds, we predict that the parental compounds host collinear antiferromagnetic states similar to those in the iron-based high temperature superconductors. The electronic physics…
The spin-fluctuation mechanism of superconductivity usually results in the presence of gapless or nodal quasiparticle states in the excitation spectrum. Nodal quasiparticle states are well established in copper-oxide, and heavy-fermion superconductors, but not in iron-based superconductors. Here, we study the pairing symmetry and mechanism of a new class of plutonium-based high-Tc superconductors and predict the presence of a nodal $s^{+-}$ wave pairing symmetry in this family. Starting from a density-functional theory (DFT) based electronic structure calculation we predict several three-dimen…
We determine the work functions of the iron arsenic compounds $A$Fe$_2$As$_2$ ($A=\mathrm{Ca, Ba, Cs}$) using photoemission spectroscopy to be 2.7 eV for CaFe$_2$As$_2$, 1.8 eV for BaFe$_2$As$_2$, and 1.3 eV for CsFe$_2$As$_2$. The work functions of these 122 iron-based superconductors track those of the elementary metal $A$ but are substantially smaller. The most likely explanation of this observation is that the cleaving surface exposes only half an $A$-layer. The low work function and good photoemission cross section of BaFe$_2$As$_2$ and CsFe$_2$As$_2$ enable photoemission even from a comm…
By performing pressure simulations within density functional theory for the family of iron-based superconductors $Ae{}A$Fe$_4$As$_4$ with $Ae$ = Ca, Sr, Ba and $A$ = K, Rb, Cs we predict in these systems the appearance of two consecutive half-collapsed tetragonal transitions at pressures $P_{c_1}$ and $P_{c_2}$, which have a different character in terms of their effect on the electronic structure. We find that, similarly to previous studies for CaKFe$_4$As$_4$, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero…
We suggest a new family of Co/Ni-based materials that may host unconventional high temperature superconductivity (high-T$_c$). These materials carry layered square lattices with each layer being formed by vertex-shared transition metal tetrahedra cation-anion complexes. The electronic physics in these materials is determined by the two dimensional layer and is fully attributed to the three near degenerated $t_{2g}$ d-orbitals close to a $d^7$ filling configuration in the d-shell of Co/Ni atoms . The electronic structure meets the necessary criteria for unconventional high T$_c$ materials propo…
Insight into the electronic structure of the pnictide family of superconductors is obtained from quantum oscillation measurements. Here we review experimental quantum oscillation data that reveal a transformation from large quasi-two dimensional electron and hole cylinders in the paramagnetic overdoped members of the pnictide family to significantly smaller three-dimensional Fermi surface sections in the antiferromagnetic parent members, via a potential quantum critical point at which an effective mass enhancement is observed. Similarities with the Fermi surface evolution from the overdoped to…
We study LiFeP, LiFeAs and NaFeAs in their paramagnetic metallic phase including dynamical electronic correlations within a density functional theory + slave-spin mean-field framework. The three compounds are found to lie next to the crossover between a normal and a Hund's metal, where a region of enhanced electronic compressibility that may boost superconductivity is systematically present in this type of systems. We find that LiFeP lies in the normal metallic regime, LiFeAs at the crossover, and NaFeAs is in the Hund's metal regime, which possibly explains the different experimental trends f…
Based on the first-principles calculations and experimental measurements, we report that the hexagonal phase of ternary transition metal pnictides TT'X (T=Zr, Hf; T'=Ru; X=P, As), which are well-known noncentrosymmetric superconductors with relatively high transition temperatures, host nontrivial bulk topology. Before the superconducting phase transition, we find that HfRuP belongs to a Weyl semimetal phase with 12 pairs of type-II Weyl points, while ZrRuAs, ZrRuP and HfRuAs belong to a topological crystalline insulating phase with trivial Fu-Kane $\mathbb Z_2$ indices but $nontrivial$ mirror …
The discovery of correlated states and superconductivity in magic-angle twisted bilayer graphene (MATBG) has established moiré quantum matter as a new platform to explore interaction-driven and topological quantum phenomena. Multitudes of phases have been realized in moiré systems, but surprisingly, robust superconductivity has been one of the least common of all, initially found in MATBG and only more recently also in magic-angle twisted trilayer graphene (MATTG). While MATBG and MATTG share some similar characteristics, they also exhibit substantial differences, such as in their response to …
Understanding the role played by broken symmetry states such as charge, spin, and orbital orders in the mechanism of emergent properties such as high-temperature superconductivity (HTSC) is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate superconductors places it at the forefront of current research. Here we show that the recently discovered BaTi$_2$Sb$_2$O superconductor and its "parent" compound BaTi$_2$As$_2$O form a symmetry-breaki…
