In order to explain the observation of an extended superconducting region in several overdoped cuprates, which contrasts the dome scenario, by means of neutron and synchrotron x-ray powder diffraction we study the crystal structure of YBa_2Cu_3O_y, where strong oxygen overdoping up to y = 7.4 is achieved under high-pressure. A bond valence sum analysis indicates that 1/5 of the extra holes created by the excess oxygen are transferred to the CuO_2 planes, thus increasing the hole density up to p=0.27 hole/Cu, where superconductivity is expected to vanish according to the dome scenario. Instead, our data confirm a previous observation [Okai, Ono and Mitsuhashi, Physica C: Superconductivity 366, 164 (2002)] that the superconducting critical temperature, T_c, remains constant with y. Our data analysis accounts for this discrepancy in terms of the much shorter bond between the apical oxygen and the planar Cu ion, which suggests that the extra holes occupy the a_1-symmetry states formed by d_3z^2-r^2 orbitals, instead of the usual b_1-symmetry Zhang-Rice singlet states formed by d_x^2-y^2 orbitals. Suitable spectroscopic measurements on single crystals may support such a two-band scenario, which would require a totally different theoretical approach to explain superconductivity in cuprates.

We report on the high-pressure synthesis of a CrS_2 phase in the form of single-crystalline nanorods. A structural refinement of Precession Electron Diffraction Tomography data confirms the nominal CrS_2 composition and unveils a ladder-type structure formed by portions of 1T-type CrS_2 layers characteristic of two-dimensional (2D) dichalcogenides connected by chains of edge-sharing CrS_6 octahedra characteristic of 3D dichalcogenides with marcasite structure. Ab initio density functional theory calculations of the relaxed structure confirm the stability of this structure and indicate a strong overlap of the 3d states of Cr with the 3p states of S, thus suggesting strong covalent Cr-S bonds and metallic behavior. Electrical resistivity, ϱ, measurements on single nanorods confirm this behavior and yield ϱ∼ 2-20 mΩ cm at 4 K. The proposed ladder-like structure of CrS_2 forms open channels along the chain direction, which may be suitable for ionic conduction.

By means of a specific heat, susceptibility and high-pressure electrical resistivity study, we show that the local magnetic moments of the intercalated V ions in V_5S_8 realize a prototype of Kondo lattice system, where an antiferromagnetic order of the moments coexists with a Fermi liquid in the VS_2 layers with intermediate heavy Fermion properties. The antiferromagnetic order and the Fermi-liquid behavior are simultaneously suppressed at a critical pressure, P_c =10 GPa, signature of a quantum critical point, which supports a Kondo lattice scenario and raises the question whether, in the paramagnetic phase at higher pressures, the heavy quasiparticles survive or form a non-Fermi liquid phase governed by the Kondo interaction.

By means of Raman scattering and infrared reflectivity spectroscopies under high pressure, we investigate the metallic phase induced by pressure at Pcr = 17.5 GPa in the Mott insulator (NaMn3)Mn4O12 (NMO). Upon approaching Pcr, two signatures of the presence of mobile carriers become increasingly pronounced: a Fano-like lineshape of the Raman modes and a Drude-like peak in the optical conductivity spectra. Remarkably, all Raman and infrared modes display a continuous evolution with pressure up to 22 GPa, indicating that no structural distortion occurs at the transition owing to the very compact quadruple perovskite structure of NMO. We therefore propose a unique scenario of a second-order Mott phase transition where only the electronic degrees of freedom are active, which opens the possibility of probing the effective Hamiltonian of the transition disentangled from the lattice degrees of freedom.

Cu K edge X-ray absorption spectra of overdoped superconducting YSr_2Cu_2.75Mo_0.25O_7.54 and Sr_2CuO_3.3 show a remarkably strong correlation of their superconductivity with the local dynamics of their Cu-Sr and Cu-apical-O pairs. This finding that the entire alkaline earth cation-apical O "dielectric" layer has an active role in the unusual electronic properties of cuprates has not been previously considered and has far reaching implications. We develop this idea of a possible role for the apical oxygen charge dynamics via a soft mode of the Sr by applying Kuramoto's synchronization technique to exact diagonalization calculations of two neighboring Cu-apical O pairs bridged by Sr and a planar O atom. These calculations show a first order phase transition to a synchronized state of the Internal Quantum Tunneling Polarons (IQTPs) in which a fraction of the hole originally confined to the apical O atoms of the cluster is transferred onto the planar O. This combination of experimental results and theory demonstrates that the Sr-O dielectric layer of cuprates most likely plays an important role in high temperature superconductivity via its collective charge dynamics that extends into the CuO_2 conducting planes.

In order to elucidate the unusual superconducting properties of cuprates in the strongly overdoped region, i.e., at hole-doping levels p≳0.4/Cu in the CuO2 plane, we study the structural and superconducting properties of a series of Cu0.75Mo0.25Sr2YCu2O7+x powder samples oxygenated under high pressure using different concentrations of KClO3 up to 35 mol %. The analysis of X-ray diffraction data indicates a high purity ∼90% of all samples and suggests that the concentration, x, of extra oxygen atoms increases with increasing KClO3 concentration. Surprisingly, the Tc values remain nearly constant within the 80–85 K range independent of KClO3 concentration, which suggests a scenario of Tc saturation. In order to account for this unexpected behaviour, we put forward the hypothesis that overdoping enhances the density of unpaired holes, which is supported by the observation of large values of the Sommerfeld coefficient in all samples. We therefore propose a scenario of electronic phase separation between normal and superconducting holes.

Time-resolved ARPES gives access to the band structure and ultrafast dynamics of excited electronic states in solids. The orbital character of the bands close to the Fermi level is essential to understand the origin of several exotic phenomena in quantum materials. By performing polarization-dependent time- and angle-resolved photoemission spectroscopy and by analyzing the photoelectron yield for two different crystal orientations, we identify the orbital character of bands below and above the chemical potential for the Dirac semimetal BaNiS_2 . Our results illustrate how the control and understanding of matrix elements’ effects in time-resolved photoemission spectroscopy can be a powerful tool for the study of quantum materials.

By means of a combined x-ray diffraction, magnetic susceptibility and specific heat study, we investigate the interplay between orthorhombic distortion and stripe-like antiferromagnetic (AFM) order in the Mott insulator BaCoS$_{2}$ at $T_N=290$ K. The data give evidence of a purely electronic AFM transition with no participation of the lattice. The observation of large thermal fluctuations in the vicinity of $T_N$ and a Schottky anomaly unveils competing ground states within a minute $\sim$1 meV energy range that differ in the orbital and spin configurations of the Co ions. This interpretation suggests that the stripe-like order results from a spontaneous symmetry breaking of the geometrically frustrated pristine tetragonal phase, which offers an ideal playground to study the driving force of multi-orbital Mott transitions without the participation of the lattice.

In the Dirac semimetal BaNiS2, the Dirac nodes are located along the Γ-M symmetry line of the Brillouin zone, instead of being pinned at fixed high-symmetry points. We take advantage of this peculiar feature to demonstrate the possibility of moving the Dirac bands along the Γ-M symmetry line in reciprocal space by varying the concentration of K atoms adsorbed onto the surface of cleaved BaNiS2 single crystals. By means of first-principles calculations, we give a full account of this observation by considering the effect of the electrons donated by the K atom on the charge transfer gap, which establishes a promising tool for engineering Dirac states at surfaces, interfaces, and heterostructures.

By means of single-crystal X-ray diffraction we have revealed the breaking of centrosymmetry when lowering the temperature under T_s = 200 K concomitantly with the setting of a commensurate superstructure in the small A-site quadruple perovskite YMn_3Mn_4O_12. This results is in agreement with all data already reported for this compounds and solve the previous inconsistency about the Yttrium position. The superstructure is characterized by the appearing of satellite reflections in the single crystal pattern, consistent with an I-centered pseudo-orthorhombic commensurate supercell with a ≈√(a_F) = 10.4352(7) Å, b ≈ 2b_F = 14.6049(9) Å, c ≈√(c_F) = 10.6961(7) Åand β = 90.110(3)^∘, where F stands for the "fundamental" high-temperature cell (a_F ≈ c_F ≈ 7.45 Å, b_F ≈ 7.34 Å, and β≈ 90^∘). The space-group was unequivocally found to be Ia, which is non polar, thus allowing for a non-zero polarization in the material. We then have investigated in detail the pyrocurrent, transport, dielectric and the DC and AC magnetic properties of polycrystalline sample of YMn_3Mn_4O_12 over a wide temperature range. These measurements clearly highlight several critical temperatures in the material and correlation between the different orders: i) the centrosymmetry is broken at high temperature (T_S = 200K), ii) then the long-range magnetic order of B-sites occurs at T_N,B = 108 K and at this same temperature the compound enters in an insulating dielectric state. iii) Finally, a remnant polarization is stabilized concomitantly with a magnetic anomaly at T* = 70 K. We propose that YMn_3Mn_4O_12 is a peculiar magnetic ferroelectric in which the polar state is driven by short-range magnetic order.

A pyrite system NiS2-xSex exhibits a bandwidth controlled Mott transition via (S,Se) substitutions in a two-step process: the antiferromagnetic insulator (AFI) to antiferromagnetic metal (AFM) transition at x similar to 0.45 followed by the AFM to paramagnetic metal (PMM) transition at x similar to 1.0. Among a few other Mott systems which exhibit similar two-step transitions, Ni(S, Se)(2) is of particular interest because a large intermediate AFM region in the phase diagram would provide unique opportunities to study the interplay between the spin and charge order. Muon spin relaxation (mu SR) measurements on NiS2-xSex have been carried out on seven different Se concentrations from x = 0 to 1.0. The results on quantum evolution demonstrate significantly random spin correlations in the AFM region associated with a rapid reduction of the average local static Ni moment size with increasing x, yet without signatures of macroscopic phase separation as confirmed by nearly full volume fraction participating in the static muon relaxation process up to x similar to 0.9 at low temperatures. The observed time spectra in the AFM region indicate Lorentzian distribution of static internal field expected for a spatially dilute spin structure. No signature of dynamic critical behavior was observed in thermal phase transitions. The previous neutron scattering studies found sharp magnetic Bragg peaks with a slower reduction of the average ordered moment size in the AFM region. By comparing and combining the muon and neutron results, here we propose a picture where the spin order is maintained by the percolation of "nonmetallic" localized and dangling Ni moments surrounded by S, while the charge transition from AFI to AFM is caused by the percolation of the conducting paths generated by the Ni-Se-Ni bonds. This model of interpenetrating charge and spin percolation captures the behavior of experimental results on (a) Se concentration for the insulator to metal transition, (b) Se concentration for the AFM to PMM transition, (c) variation of Hall effect in the AFM region due to conducting Ni charges on the backbone of the percolating charge network, (d) evolution of the neutron Bragg intensity, (e) evolution of the muon static local fields, and (f) spatial variation of the local conductance observed by STM.

Time-resolved ARPES gives access to the band structure and ultrafast dynamics of excited electronic states in solids. The orbital character of the bands close to the Fermi level is essential to understand the origin of several exotic phenomena in quantum materials. By performing polarization dependent time- and angle-resolved photoemission spectroscopy and by analyzing the chirality of the photoelectron yield for two different crystal orientations, we identify the orbital character of bands below and above the chemical potential for the Dirac semimetal BaNiS2. Our results illustrate how the control and understanding of matrix elements effects in time-resolved photoemission spectroscopy can be a powerful tool for the study of quantum materials.

By means of in situ synchrotron X-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the structural stability of the quadruple perovskite LaMn7O12. At 34 GPa, the data unveil a first-order structural phase transition from monoclinic I2/m to cubic Im-3 symmetry characterized by a pronounced contraction of the unit cell and by a significant modifications in the Raman phonon modes. The phase transition is also marked by the suppression of Jahn-Teller distortion which is present in the ambient monoclinic phase. In addition, above 20 GPa pressure, a sudden and simultaneous broadening is observed in several Raman modes which suggests the onset of a sizable electron-phonon interaction and incipient charge mobility. Considering that LaMn7O12 is paramagnetic insulator at ambient, and Jahn-Teller distortion is frozen in the high-pressure Im-3 phase, we argue that this phase could be a potential candidate to host a purely electronic insulator-metal transition with no participation of the lattice

BaNiS$_2$ is a system dominated by a fourfold Dirac-cone network. We measured the optical conductivity and Landau level spectra of the Dirac nodal lines and quantitatively modeled the response through ab initio calculations. The optical conductivity shows a highly unusual temperature-independent isosbestic line. Magneto-optical spectra show a nearly $\sqrt{B}$ behavior, the hallmark of a conical dispersion. BaNiS$_2$ is a simple prototype of a Dirac nodal line semimetal: its Dirac nodal lines are only slightly gapped, and they disperse exclusively along the out-of-plane direction. Our first-principles calculations account for the observed isosbestic conductivity, which terminates in a Van Hove singularity. We argue that such an optical response is universal for Dirac cones, which must connect with each other within the Brillouin zone by breaking their conical shape.

We investigate the optical conductivity and far-infrared magneto-optical response of BaNiS$_2$, a simple square-lattice semimetal characterized by Dirac nodal lines that disperse exclusively along the out-of-plane direction. With the magnetic field aligned along the nodal line the in-plane Landau level spectra show a nearly $\sqrt{B}$ behavior, the hallmark of a conical-band dispersion with a small spin-orbit coupling gap. The optical conductivity exhibits an unusual temperature-independent isosbestic line, ending at a Van Hove singularity. First-principles calculations unambiguously assign the isosbestic line to transitions across Dirac nodal states. Our work suggests a universal topology of the electronic structure of Dirac nodal lines.

Dirac fermions play a central role in the study of topological phases, for they can generate a variety of exotic states, such as Weyl semimetals and topological insulators. The control and manipulation of Dirac fermions constitute a fundamental step towards the realization of novel concepts of electronic devices and quantum computation. By means of ARPES experiments and ab initio simulations, here we show that Dirac states can be effectively tuned by doping a transition metal sulfide, BaNiS2, through Co/Ni substitution. The symmetry and chemical characteristics of this material, combined with the modification of the charge transfer gap of BaCo_{1-x}Ni_{x}S_{2} across its phase diagram, lead to the formation of Dirac lines whose position in k-space can be displaced along the Gamma M symmetry direction, and their form reshaped. Not only does the doping x tailor the location and shape of the Dirac bands, but it also controls the metal-insulator transition in the same compound, making BaCo_{1-x}Ni_{x}S_{2} a model system to functionalize Dirac materials by varying the strength of electron correlations.

By means of high-pressure resistivity measurements on single crystals, we investigate the charge transport properties of ${\mathrm{Cu}}_{x}{\mathrm{PdTe}}_{2}$, notable for the combination of topological type-II Dirac semimetallic properties with superconductivity up to ${T}_{c}=2.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In both pristine $(x=0)$ and intercalated $(x=0.05)$ samples, we find an unconventional ${T}^{4}$ power law behavior of the low-temperature resistivity visible up to $\ensuremath{\sim}40$ K and remarkably stable under pressure up to 8.2 GPa. This observation is explained by the low carrier density $n$, which strongly reduces the $k$ region available for electron-phonon scattering, as previously reported in other low-$n$ two-dimensional systems, such as multilayer graphene and semiconductor heterostructures. Our data analysis complemented by specific heat measurements and supported by previous quantum oscillation studies and ab initio calculations suggests a scenario of one-band charge transport. Within this scenario, our analysis yields a large value of the transport electron-phonon coupling constant ${\ensuremath{\lambda}}_{tr}=1.2$ at ambient pressure that appears to be strongly enhanced by pressure assuming a constant effective mass.

Dirac fermions play a central role in the study of topological phases, for they can generate a variety of exotic states, such as Weyl semimetals and topological insulators. The control and manipulation of Dirac fermions constitute a fundamental step toward the realization of novel concepts of electronic devices and quantum computation. By means of Angle-Resolved Photo-Emission Spectroscopy (ARPES) experiments and ab initio simulations, here, we show that Dirac states can be effectively tuned by doping a transition metal sulfide, [Formula: see text], through Co/Ni substitution. The symmetry and chemical characteristics of this material, combined with the modification of the charge-transfer gap of [Formula: see text] across its phase diagram, lead to the formation of Dirac lines, whose position in k-space can be displaced along the [Formula: see text] symmetry direction and their form reshaped. Not only does the doping x tailor the location and shape of the Dirac bands, but it also controls the metal-insulator transition in the same compound, making [Formula: see text] a model system to functionalize Dirac materials by varying the strength of electron correlations.

By means of diffuse x-ray scattering (DS) and inelastic x-ray scattering (IXS), we probe directly the chargeordering (CO) dynamics in the Verwey system (NaMn3)Mn4O12, where a peculiar quadruple perovskite structure with no oxygen disorder stabilizes a nearly full Mn3+/Mn4+ static charge order at TCO = 175 K concomitant to a commensurate structural modulation with propagation vector qCO = (21, 12, 0). At TCO, the IXS spectra unveil a softening of a 35.3-meV phonon at qCO. Lattice dynamical calculations enable us to attribute this soft phonon to an Ag mode whose polarization matches the Jahn-Teller-like distortion pattern of the structural modulation. This result demonstrates that the Jahn-Teller instability is the driving force of the CO Verwey transition in (NaMn3)Mn4O12, thus elucidating a long-standing controversy regarding the mechanism of this transition observed in other mixed-valence systems like magnetite.