Using angle-resolved photoemission spectroscopy, combined with first principle and coupled self-consistent Poisson-Schrödinger calculations, we demonstrate that potassium (K) atoms adsorbed on the low-temperature phase of 1T-TiSe_{2} induce the creation of a two-dimensional electron gas (2DEG) and quantum confinement of its charge-density wave (CDW) at the surface. By further changing the K coverage, we tune the carrier density within the 2DEG that allows us to nullify, at the surface, the electronic energy gain due to exciton condensation in the CDW phase while preserving a long-range structural order. Our Letter constitutes a prime example of a controlled exciton-related many-body quantum state in reduced dimensionality by alkali-metal dosing.

Explaining the mechanism of superconductivity in the high-$T_c$ cuprates requires an understanding of what causes electrons to form Cooper pairs. Pairing can be mediated by phonons, the screened Coulomb force, spin or charge fluctuations, excitons, or by a combination of these. An excitonic pairing mechanism has been postulated, but experimental evidence for coupling between conduction electrons and excitons in the cuprates is sporadic. Here we use resonant inelastic x-ray scattering (RIXS) to monitor the temperature dependence of the $\underline{d}d$ exciton spectrum of Bi$_2$Sr$_2$CaCu$_2$O$_{8-x}$ (Bi-2212) crystals with different charge carrier concentrations. We observe a significant change of the $\underline{d}d$ exciton spectra when the materials pass from the normal state into the superconductor state. Our observations show that the $\underline{d}d$ excitons start to shift up (down) in the overdoped (underdoped) sample when the material enters the superconducting phase. We attribute the superconductivity-induced effect and its sign-reversal from underdoped to overdoped to the exchange coupling of the site of the $\underline{d}d$ exciton to the surrounding copper spins.

Doped strontium titanate SrTiO3 (STO) is one of the most dilute superconductors known today. The fact that superconductivity occurs at very low carrier concentrations is one of the two reasons that the pairing mechanism is not yet understood, the other is the role played by the proximity to a ferroelectric instability. In undoped STO, ferroelectric order can in fact be stabilized by substituting 16O with its heavier isotope 18O. Here we explore the superconducting properties of doped and isotope-substituted SrTi(18O16 y O1−y)3−δ for 0 ≤ y ≤ 0.81 and carrier concentrations between 6×1017 and 2 × 1020 cm−3 (δ < 0.02). We show that the superconducting Tc increases when the 18O concentration is increased. For carrier concentrations around 5×1019 cm−3 this Tc increase amounts to almost a factor 3, with Tc as high as 580 mK for y = 0.74. When approaching SrTiO3 the maximum Tc occurs at a much smaller carrier densities than for pure SrTiO3. Our observations agree qualitatively with a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft mode.

Explaining the mechanism of superconductivity in the high-$T_c$ cuprates requires an understanding of what causes electrons to form Cooper pairs. Pairing can be mediated by phonons, the screened Coulomb force, spin or charge fluctuations, excitons, or by a combination of these. An excitonic pairing mechanism has been postulated, but experimental evidence for coupling between conduction electrons and excitons in the cuprates is sporadic. Here we use resonant inelastic x-ray scattering (RIXS) to monitor the temperature dependence of the $\underline{d}d$ exciton spectrum of Bi$_2$Sr$_2$CaCu$_2$O$_{8-x}$ (Bi-2212) crystals with different charge carrier concentrations. We observe a significant change of the $\underline{d}d$ exciton spectra when the materials pass from the normal state into the superconductor state. From theoretical modeling, we determine the strength of the coupling between the electrons and the excitons. Our observations show that the coupling to excitons can be strong enough to play an important role in stabilizing the superconducting state.

Sr2TiO4, first member of the Ruddlesden-Popper series Srn+1TinO3n+1, has been long known to undergo a phase transition at 1550 °C.This transition makes the growth of single crystals of this material highly challenging, because it usually breaks the crystal into a periodic array of uneven lamellae.While the low temperature tetragonal phase is widely studied due to its close connection to the famous perovskite SrTiO3, there is little information about the high temperature α-phase, except for an unindexed powder pattern by Drys&Trzebiatowski [1].We stabilized the high-temperature α-Sr2TiO4 crystals by rapid cooling of the incongruent melt from above the liquidus temperature.The α-phase crystallizes in the orthorhombic Pna21 group with lattice parameters a=14.2901(5)Å b=5.8729(2)Å c=10.0872(3)Å and is isostructural to the orthorhombic forms of Sr2VO4 and Sr2CrO4 (which belong to the β-K2SO4 structure type).Its structure is formed by a complicated framework of large SrOx polyhedra with tetrahedral cavities occupied by the transition metal.The tetrahedral coordination of Ti IV makes the α-Sr2TiO4 quite a rare case among titanate compounds, the only other known example being the barium orthotitanate Ba2TiO4 [2].However, whereas in Ba2TiO4 the coordination is tetrahedral in both high-and low-temperature polymorphs and the topotactic relation between the two is known, in the case of Sr2TiO4 a transition occurs to the layered Ruddlesden-Popper structure with octahedral titanium coordination.In this work, we report for the first time the crystal structure of the high-temperature α-phase of Sr2TiO4.We elucidate the structural differences between the related compounds and discuss possible mechanism driving the structural transition.

F. Barantani,1, 2 M. K. Tran,1 I. Madan,2 I. Kapon,1 N. Bachar,1 A. Bercher,1 T. C. Asmara,3 E. Paris,3 Y. Tseng,3 W. Zhang,3 Y. Hu,4, 5 X. X. Huang,4, 5 E. Giannini,1 G. Gu,6 T. P. Devereaux,5, 7, 8 C. Berthod,1 F. Carbone,2 T. Schmitt,3 and D. van der Marel1 1Department of Quantum Matter Physics, University of Geneva, 1211 Geneva, Switzerland 2Institute of Physics, École Polytechnique Fédérale de Lausanne, Lausanne, 1015, Switzerland 3Photon Science Division, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland 4Department of Applied Physics, Stanford University, CA 94305, USA 5Stanford Institute for Materials and Energy Sciences, SLAC,CA 94025, USA 6Brookhaven National Laboratory, Upton, NY 11973 5000, USA 7Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA 8Geballe Laboratory for Advanced Materials, Stanford University, CA 94305, USA (Dated: August 16, 2021)

Optical selection rules in monolayers of transition metal dichalcogenides and of their heterostructures are determined by the conservation of the z-component of the total angular momentum-J(Z)= L-Z+S-Z - associated with the C-3 rotational lattice symmetry which assumes half integer values corresponding, modulo 3, to distinct states. Here we show, based on polarization resolved and low temperature magneto-optical spectroscopy experiments, that the conservation of the total angular momentum in these systems leads to a very efficient exciton-phonon interaction when the coupling is mediated through chiral phonons. We identify these phonons as the Gamma point E" modes which despite carrying angular momentum +/- 1 are able to induce an excitonic spin-flip of -/+ 2 thanks to the C(3)symmetry. These experiments reveal the crucial role of electron-phonon interaction in the carrier dynamics of group 6 transition metal dichalcogenides.

The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present between the upper-critical-field values measured in MgB2 polycrystalline bulk superconductors and those of thin films, where values as high as ~ 50 T have been achieved at 4.2 K. Filling this gap would unlock the potential of MgB2 for magnet applications. This work presents the results of an extensive experimental campaign on MgB2 bulk samples, which has been guided by a Design of Experiment. We modeled the dependence of the upper critical field on the main synthesis parameters and established a new record (~ 35 T at 4.2 K) preparing C-doped bulk samples by a non-conventional rapid-synthesis route. This value appears to be an upper boundary for the upper critical field in bulk samples. Structural disorder in films seems to act selectively on one of the two bands where superconductivity in MgB2 takes place: this enhances the upper critical field while reducing the critical temperature only by few Kelvins. On the other hand, the critical temperature in bulk samples decreases monotonically when structural disorder increases, and this imposes a limit to the maximum achievable upper critical field.

We report high-resolution angle resolved photoemission measurements on single crystals of Pt2HgSe3 grown by high-pressure synthesis. Our data reveal a gapped Dirac nodal line whose (001)-projection separates the surface Brillouin zone in topological and trivial areas. In the non-trivial $k$-space range we find surface states with multiple saddle-points in the dispersion resulting in two van Hove singularities in the surface density of states. Based on density functional theory calculations, we identify these surface states as signatures of a topological crystalline state which coexists with a weak topological phase.

The transition metal dichalcogenide $1T$-TiSe$_2$ is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge density wave (CDW) at a critical temperature T$_{c}\approx 200$K. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the non-reconstructed Fermi surface topology, remains elusive. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we investigate $1T$-TiSe$_{2-x}$S$_x$ single crystals. Using STM, we first show that the long-range phase coherent CDW state is stable against S substitutions with concentrations at least up to $x=0.34$. The ARPES measurements then reveal a slow but continuous decrease of the overlap between the electron and hole ($e$-$h$) bands of the semimetallic normal-state well reproduced by DFT and related to slight reductions of both the CDW order parameter and $T_c$. Our DFT calculations further predict a semimetal-to-semiconductor transition of the normal state at a higher critical S concentration of $x_c$=0.9 $\pm$0.1, that coincides with a melted CDW state in TiSeS as measured with STM. Finally, we rationalize the $x$-dependence of the $e$-$h$ band overlap in terms of isovalent substitution-induced competing chemical pressure and charge localization effects. Our study highlights the key role of the $e$-$h$ band overlap for the CDW instability.

The discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has generated widespread interest. Making further progress in this area requires quantitative knowledge of the magnetic properties of vdW magnets at the nanoscale. We used scanning single-spin magnetometry based on diamond nitrogen-vacancy centers to image the magnetization, localized defects, and magnetic domains of atomically thin crystals of the vdW magnet chromium(III) iodide (CrI 3 ). We determined the magnetization of CrI 3 monolayers to be ≈16 Bohr magnetons per square nanometer, with comparable values in samples with odd numbers of layers; however, the magnetization vanishes when the number of layers is even. We also found that structural modifications can induce switching between ferromagnetic and antiferromagnetic interlayer ordering. These results demonstrate the benefit of using single-spin scanning magnetometry to study the magnetism of 2D vdW magnets.

The semimetallic or semiconducting nature of the transition metal dichalcogenide 1T-TiSe_2 remains under debate after many decades mainly due to the fluctuating nature of its 2 × 2 × 2 charge-density-wave (CDW) phase at room-temperature. In this letter, using angle-resolved photoemission spectroscopy, we unambiguously demonstrate that the 1T-TiSe_2 normal state is semimetallic with an electron-hole band overlap of ∼110 meV by probing the low-energy electronic states of the perturbed CDW phase strongly doped by alkali atoms. Our study not only closes a long-standing debate but also supports the central role of the Fermi surface for driving the CDW and superconducting instabilities in 1T-TiSe_2.

Spatially inhomogeneous electronic states are expected to be key ingredients for the emergence of superconducting phases in quantum materials hosting charge-density-waves (CDWs). Prototypical materials are transition-metal dichalcogenides (TMDCs) and among them, 1$T$-TiSe$_2$ exhibiting intertwined CDW and superconducting states under Cu intercalation, pressure or electrical gating. Although it has been recently proposed that the emergence of superconductivity relates to CDW fluctuations and the development of spatial inhomogeneities in the CDW order, the fundamental mechanism underlying such a phase separation (PS) is still missing. Using angle-resolved photoemission spectroscopy and variable-temperature scanning tunneling microscopy, we report on the phase diagram of the CDW in 1$T$-TiSe$_2$ as a function of Ti self-doping, an overlooked degree of freedom inducing CDW texturing. We find an intrinsic tendency towards electronic PS in the vicinity of Fermi surface (FS) "hot spots", i.e. locations with band crossings close to, but not at the Fermi level. We therefore demonstrate an intimate relationship between the FS topology and the emergence of spatially textured electronic phases which is expected to be generalizable to many doped CDW compounds.

In quantum materials, doping plays a central role in determining the electronic ground state among competing phases such as charge order and superconductivity. In addition, the real-space distribution of dopants has been found to induce quenched disorder and lead to phase separation (PS). However, an open question remains as to the extent to which native low-concentration defects on the nanoscale can affect macroscopic electronic phase behavior. An important materials class exhibiting phase diagrams including intertwined charge density waves (CDWs), Mott states and superconductivity is the family of layered transition-metal dichalcogenides (TMDCs). Among them, $1T$-TiSe$_2$ is a prototypical system hosting mixed CDW-superconducting phases under Cu intercalation, pressure or electrical gating, the origin of which remains elusive due to the still-debated nature of the CDW. Here, we combine angle-resolved photoemission spectroscopy (ARPES) and variable-temperature scanning tunnelling microscopy (VT-STM) to obtain complementary momentum and real-space insights into the CDW phase transition in $1T$-TiSe$_2$ crystals natively doped by low concentrations of Ti intercalants. We report persistent nanoscale PS in an extended region of the phase diagram and demonstrate that it results from a combination of Coulomb frustration arising from a doping-driven topological Lifshitz transition and quenched lattice disorder. Our study highlights the intimate relationship between electronic PS and intrinsic impurities and reveals that this overlooked degree of freedom is the tuning knob of the spatially inhomogeneous electronic landscape hosting the superconducting state of $1T$-TiSe$_2$.

Spatially inhomogeneous electronic states are expected to be key ingredients for the emergence of superconducting phases in quantum materials hosting charge-density-waves (CDWs). Prototypical materials are transition-metal dichalcogenides (TMDCs) and among them, 1$T$-TiSe$_2$ exhibiting intertwined CDW and superconducting states under Cu intercalation, pressure or electrical gating. Although it has been recently proposed that the emergence of superconductivity relates to CDW fluctuations and the development of spatial inhomogeneities in the CDW order, the fundamental mechanism underlying such a phase separation (PS) is still missing. Using angle-resolved photoemission spectroscopy and variable-temperature scanning tunneling microscopy, we report on the phase diagram of the CDW in 1$T$-TiSe$_2$ as a function of Ti self-doping, an overlooked degree of freedom inducing CDW texturing. We find an intrinsic tendency towards electronic PS in the vicinity of Fermi surface (FS) hot spots, i.e. locations with band crossings close to, but not at the Fermi level. We therefore demonstrate an intimate relationship between the FS topology and the emergence of spatially textured electronic phases which is expected to be generalizable to many doped CDW compounds.

The transition metal dichalcogenide 1T-TiSe_{2}-two-dimensional layered material undergoing a commensurate 2×2×2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ≈200  K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe the three-dimensional character of the PLD. Furthermore, the inversion-symmetric achiral nature of the CDW in the z direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral order. This study paves the way to a simultaneous real-space probing of both charge and structural reconstructions in CDW compounds.

B. Hildebrand, ∗ T. Jaouen, † M.-L. Mottas, G. Monney, C. Barreteau, E. Giannini, D. R. Bowler, and P. Aebi Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, CH-1700 Fribourg, Switzerland Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK (Dated: July 6, 2018)

1T -TiSe2−xSx single crystals M.-L. Mottas, ∗ T. Jaouen, † B. Hildebrand, M. Rumo, F. Vanini, E. Razzoli, 3 E. Giannini, C. Barreteau, D. R. Bowler, C. Monney, H. Beck, and P. Aebi Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, CH-1700 Fribourg, Switzerland Quantum Matter Institute, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 Departement of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada V6T 1Z1 Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK (Dated: December 6, 2018)