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The specific heat $C$ of the cuprate superconductors ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ and ${\mathrm{Bi}}_{2+y}{\mathrm{Sr}}_{2\ensuremath{-}x\ensuremath{-}y}{\mathrm{La}}_{x}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ was measured at low temperatures (down to 0.5 K) for dopings $p$ close to ${p}^{★}$, the critical doping for the onset of the pseudogap phase. A magnetic field up to 35 T was applied to suppress superconductivity, giving direct access to the normal state at low temperatures, and enabling a determination of ${C}_{\mathrm{e}}$, the electronic contribution to the normal-state specific heat at $T\ensuremath{\rightarrow}0$. In ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ at $x=p=0.22$, 0.24 and 0.25, ${C}_{\mathrm{e}}/T=15\phantom{\rule{0.28em}{0ex}}\mathrm{to}\phantom{\rule{0.28em}{0ex}}16\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$ at $T=2\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, values that are twice as large as those measured at higher doping ($pg0.3$) and lower doping ($pl0.15$). This confirms the presence of a broad peak in the doping dependence of ${C}_{\mathrm{e}}$ at ${p}^{★}\ensuremath{\simeq}0.19$ as previously reported for samples in which superconductivity was destroyed by Zn impurities. Moreover, at those three dopings, we find a logarithmic growth as $T\ensuremath{\rightarrow}0$ such that ${C}_{\mathrm{e}}/T\ensuremath{\sim}B\phantom{\rule{0.16em}{0ex}}ln({T}_{0}/T)$. The peak versus $p$ and the logarithmic dependence versus $T$ are the two typical thermodynamic signatures of quantum criticality. In the very different cuprate ${\mathrm{Bi}}_{2+y}{\mathrm{Sr}}_{2\ensuremath{-}x\ensuremath{-}y}{\mathrm{La}}_{x}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$, we again find that ${C}_{\mathrm{e}}/T\ensuremath{\sim}B\phantom{\rule{0.16em}{0ex}}ln({T}_{0}/T)$ at $p\ensuremath{\simeq}{p}^{★}$, strong evidence that this $ln(1/T)$ dependence of the electronic specific heat---first discovered in the cuprates ${\mathrm{La}}_{1.8\ensuremath{-}x}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ and ${\mathrm{La}}_{1.6\ensuremath{-}x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$---is a universal property of the pseudogap critical point.

Electron-phonon and electron-electron interactions are found to cooperatively drive the long-range charge order in cuprates., Charge order is universal to all hole-doped cuprates. Yet, the driving interactions remain an unsolved problem. Electron-electron interaction is widely believed to be essential, whereas the role of electron-phonon interaction is unclear. We report an ultrahigh-resolution resonant inelastic x-ray scattering (RIXS) study of the in-plane bond-stretching phonon mode in stripe-ordered cuprate La1.675Eu0.2Sr0.125CuO4. Phonon softening and lifetime shortening are found around the charge ordering wave vector. In addition to these self-energy effects, the electron-phonon coupling is probed by its proportionality to the RIXS cross section. We find an enhancement of the electron-phonon coupling around the charge-stripe ordering wave vector upon cooling into the low-temperature tetragonal structure phase. These results suggest that, in addition to electronic correlations, electron-phonon coupling contributes substantially to the emergence of long-range charge-stripe order in cuprates.

As exemplified by the growing interest in the quantum anomalous Hall effect, the research on topology as an organizing principle of quantum matter is greatly enriched from the interplay with magnetism. In this vein, we present a combined electrical and thermoelectrical transport study on the magnetic Weyl semimetal ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$. Unconventional contribution to the anomalous Hall and anomalous Nernst effects were observed both above and below the magnetic transition temperature of ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$, indicating the existence of significant Berry curvature. ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ represents a rare case in which this unconventional transverse transport emerges both above and below the magnetic transition temperature in the same material. The transport properties evolve with temperature and field in the antiferromagnetic phase in a different manner than in the paramagnetic phase, suggesting different mechanisms to their origin. Our results indicate ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ is a fertile playground for investigating the interplay between magnetism and topology, and potentially a plethora of topologically nontrivial phases rooted in this interplay.

Tuning of electronic density-of-states singularities is a common route to unconventional metal physics. Conceptually, van Hove singularities are realized only in clean two-dimensional systems. Little attention has therefore been given to the disordered (dirty) limit. Here, we provide a magnetotransport study of the dirty metamagnetic system calcium-doped strontium ruthenate. Fermi liquid properties persist across the metamagnetic transition, but with an unusually strong variation of the Kadowaki-Woods ratio. This is revealed by a strong decoupling of inelastic electron scattering and electronic mass inferred from density-of-state probes. We discuss this Fermi liquid behavior in terms of a magnetic field tunable van Hove singularity in the presence of disorder. More generally, we show how dimensionality and disorder control the fate of transport properties across metamagnetic transitions., Communications Physics, 4 (1)

We present a combined oxygen $K$-edge x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) study of the bilayer ruthenate ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$. Our RIXS experiments on ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ were carried out on the overlapping planar and interplanar oxygen resonances, which are distinguishable from the apical one. Comparison to equivalent oxygen $K$-edge spectra recorded on band-Mott insulating ${\mathrm{Ca}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ is made. In contrast to ${\mathrm{Ca}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ spectra, which contain excitations linked to Mott physics, ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ spectra feature only intra-${t}_{2g}$ ones that do not directly involve the Coulomb energy scale. As found in ${\mathrm{Ca}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$, we resolve two intra-${t}_{2g}$ excitations in ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$. Moreover, the lowest lying excitation in ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ shows a significant dispersion, revealing a collective character different from what is observed in ${\mathrm{Ca}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$. Theoretical modeling supports the interpretation of this lowest energy excitation in ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ as a magnetic transverse mode with multiparticle character, whereas the corresponding excitation in ${\mathrm{Ca}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ is assigned to combined longitudinal and transverse spin modes. These fundamental differences are discussed in terms of the inequivalent magnetic ground-state manifestations in ${\mathrm{Ca}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ and ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$.

The nature of the pseudogap phase of hole-doped cuprate superconductors is still not understood fully. Several experiments have suggested that this phase ends at a critical hole doping level p*, but the nature of the ground state for lower doping is still debated. Here, we use local nuclear magnetic resonance and bulk ultrasound measurements to show that, once competing effects from superconductivity are removed by high magnetic fields, the spin-glass phase of La2−xSrxCuO4 survives up to a doping level consistent with p*. In this material, the antiferromagnetic-glass phase extends from the doped Mott insulator at p = 0.02 up to p* ≈ 0.19, which provides a connection between the pseudogap and the physics of the Mott insulator. Furthermore, the coincidence of the pseudogap boundary with a magnetic quantum phase transition in the non-superconducting ground state has implications for the interpretation of other experiments, particularly for transport and specific-heat measurements in high magnetic fields. NMR and ultrasound measurements show that the spin-glass phase exists in a cuprate all the way up to the doping that marks the end of the pseudogap phase. This highlights the possible connection between the pseudogap and Mott physics.

We use resonant inelastic x-ray scattering to investigate charge-stripe correlations in La_{1.675}Eu_{0.2}Sr_{0.125}CuO_{4}. By differentiating elastic from inelastic scattering, it is demonstrated that charge-stripe correlations precede both the structural low-temperature tetragonal phase and the transport-defined pseudogap onset. The scattering peak amplitude from charge stripes decays approximately as T^{-2} towards our detection limit. The in-plane integrated intensity, however, remains roughly temperature independent. Therefore, although the incommensurability shows a remarkably large increase at high temperature, our results are interpreted via a single scattering constituent. In fact, direct comparison to other stripe-ordered compounds (La_{1.875}Ba_{0.125}CuO_{4}, La_{1.475}Nd_{0.4}Sr_{0.125}CuO_{4}, and La_{1.875}Sr_{0.125}CuO_{4}) suggests a roughly constant integrated scattering intensity across all these compounds. Our results therefore provide a unifying picture for the charge-stripe ordering in La-based cuprates. As charge correlations in La_{1.675}Eu_{0.2}Sr_{0.125}CuO_{4} extend beyond the low-temperature tetragonal and pseudogap phase, their emergence heralds a spontaneous symmetry breaking in this compound.

By using mostly the muon-spin rotation/relaxation ($\ensuremath{\mu}\mathrm{SR}$) technique, we investigate the superconductivity (SC) of ${\mathrm{Nb}}_{5}{\mathrm{Ir}}_{3\ensuremath{-}x}{\mathrm{Pt}}_{x}\mathrm{O}$ ($x=0$ and 1.6) alloys, with ${T}_{c}=10.5$ and 9.1 K, respectively. At a macroscopic level, their superconductivity was studied by electrical resistivity, magnetization, and specific-heat measurements. In both compounds, the electronic specific heat and the low-temperature superfluid density data suggest a nodeless SC. The superconducting gap value and the specific heat discontinuity at ${T}_{c}$ are larger than that expected from BCS theory in the weak-coupling regime, indicating strong-coupling superconductivity in the ${\mathrm{Nb}}_{5}{\mathrm{Ir}}_{3\ensuremath{-}x}{\mathrm{Pt}}_{x}\mathrm{O}$ family. In ${\mathrm{Nb}}_{5}{\mathrm{Ir}}_{3}\mathrm{O}$, multigap SC is evidenced by the field dependence of the electronic specific heat coefficient and the superconducting Gaussian relaxation rate, as well as by the temperature dependence of the upper critical field. Pt substitution suppresses one of the gaps, and ${\mathrm{Nb}}_{5}{\mathrm{Ir}}_{1.4}{\mathrm{Pt}}_{1.6}\mathrm{O}$ becomes a single-gap superconductor. By combining our extensive experimental results, we provide evidence for a multiple- to single-gap SC crossover in the ${\mathrm{Nb}}_{5}{\mathrm{Ir}}_{3\ensuremath{-}x}{\mathrm{Pt}}_{x}\mathrm{O}$ family.

A comprehensive angle-resolved photoemission spectroscopy study of the band structure in single-layer cuprates is presented with the aim of uncovering universal trends across different materials. Five different hole- and electron-overdoped cuprate superconductors (${\mathrm{La}}_{1.59}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.21}{\mathrm{CuO}}_{4}$, ${\mathrm{La}}_{1.77}{\mathrm{Sr}}_{0.23}{\mathrm{CuO}}_{4}$, ${\mathrm{Bi}}_{1.74}{\mathrm{Pb}}_{0.38}{\mathrm{Sr}}_{1.88}{\mathrm{CuO}}_{6+{\delta}}$, ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6+{\delta}}$, and ${\mathrm{Pr}}_{1.15}{\mathrm{La}}_{0.7}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4}$) have been studied with special focus on the bands with a predominately $d$-orbital character. Using a light polarization analysis, the ${e}_{g}$ and ${t}_{2g}$ bands are identified across these materials. A clear correlation between the ${d}_{3{z}^{2}{-}{r}^{2}}$ band energy and the apical oxygen distance ${d}_{\mathrm{A}}$ is demonstrated. Moreover, the compound dependence of the ${d}_{{x}^{2}{-}{y}^{2}}$ band bottom and the ${t}_{2g}$ band top is revealed. A direct comparison to density functional theory (DFT) calculations employing hybrid exchange-correlation functionals demonstrates excellent agreement. We thus conclude that the DFT methodology can be used to describe the global band structure of overdoped single-layer cuprates on both the hole- and electron-doped side." name="description" />

Electronic band structures in solids stem from a periodic potential reflecting the structure of either the crystal lattice or electronic order. In the stoichiometric ruthenate Ca3Ru2O7, numerous Fermi surface-sensitive probes indicate a low-temperature electronic reconstruction. Yet, the causality and the reconstructed band structure remain unsolved. Here, we show by angle-resolved photoemission spectroscopy, how in Ca3Ru2O7 a C2-symmetric massive Dirac semimetal is realized through a Brillouin-zone preserving electronic reconstruction. This Dirac semimetal emerges in a two-stage transition upon cooling. The Dirac point and band velocities are consistent with constraints set by quantum oscillation, thermodynamic, and transport experiments, suggesting that the complete Fermi surface is resolved. The reconstructed structure—incompatible with translational-symmetry-breaking density waves—serves as an important test for band structure calculations of correlated electron systems.

We present a soft x-ray angle-resolved photoemission spectroscopy study of the overdoped high-temperature superconductors La$_{2-x}$Sr$_x$CuO$_4$ and La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$. In-plane and out-of-plane components of the Fermi surface are mapped by varying the photoemission angle and the incident photon energy. No $k_z$ dispersion is observed along the nodal direction, whereas a significant antinodal $k_z$ dispersion is identified. Based on a tight-binding parametrization, we discuss the implications for the density of states near the van-Hove singularity. Our results suggest that the large electronic specific heat found in overdoped La$_{2-x}$Sr$_x$CuO$_4$ can not be assigned to the van-Hove singularity alone. We therefore propose quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat.

Resonant inelastic X-ray scattering (RIXS) experiments performed at the oxygen-$K$ edge on the iridate perovskites {\SIOS} and {\SION} reveal a sequence of well-defined dispersive modes over the energy range up to $\sim 0.8$ eV. The momentum dependence of these modes and their variation with the experimental geometry allows us to assign each of them to specific collective magnetic and/or electronic excitation processes, including single and bi-magnons, and spin-orbit and electron-hole excitons. We thus demonstrated that dispersive magnetic and electronic excitations are observable at the O-$K$ edge in the presence of the strong spin-orbit coupling in the $5d$ shell of iridium and strong hybridization between Ir $5d$ and O $2p$ orbitals, which confirm and expand theoretical expectations. More generally, our results establish the utility of O-$K$ edge RIXS for studying the collective excitations in a range of $5d$ materials that are attracting increasing attention due to their novel magnetic and electronic properties. Especially, the strong RIXS response at O-$K$ edge opens up the opportunity for investigating collective excitations in thin films and heterostructures fabricated from these materials., Comment: 6 pages, 4 figures with supplementary material. Accepted by PRB Rapid Communications

Phase transitions and symmetry are intimately linked. Melting of ice, for example, restores translation invariance. The mysterious hidden order (HO) phase of URu$_2$Si$_2$ has, despite relentless research efforts, kept its symmetry breaking element intangible. Here we present a high-resolution x-ray diffraction study of the URu$_2$Si$_2$ crystal structure as a function of hydrostatic pressure. Below a critical pressure threshold $p_c\approx3$ kbar, no tetragonal lattice symmetry breaking is observed even below the HO transition $T_{HO}=17.5$ K. For $p>p_c$, however, a pressure-induced rotational symmetry breaking is identified with an onset temperatures $T_{OR}\sim 100$ K. The emergence of an orthorhombic phase is found and discussed in terms of an electronic nematic order that appears unrelated to the HO, but with possible relevance for the pressure-induced antiferromagnetic (AF) phase. Existing theories describe the HO and AF phases through an adiabatic continuity of a complex order parameter. Since none of these theories predicts a pressure-induced nematic order, our finding adds an additional symmetry breaking element to this long-standing problem., 6 pages, 4 figures and supplemental materials

We present a comprehensive study of how superconducting fluctuations in the normal state contribute to the conductivity tensor in a thin (119 $\AA$) film of NbN. It is shown how these fluctuations drive a sign change in the Hall coefficient $R_\mathrm{H}$ for low magnetic fields near the superconducting transition. The scaling behaviours as a function of distance to the transition $\epsilon=\ln(T/T_\mathrm{c})$ of the longitudinal ($\sigma_\mathrm{xx}$) and transverse ($\sigma_\mathrm{xy}$) conductivity is found to be consistent with Gaussian fluctuation theory. Moreover, excellent quantitative agreement between theory and experiment is obtained without any adjustable parameters. Our experimental results thus provide a case study of the conductivity tensor originating from short-lived Cooper pairs., Comment: 6 pages, 4 figures

The application of large magnetic fields ($B \sim B_{c2}$) to layered cuprates suppresses their high temperature superconducting behaviour and reveals competing ground states. In the widely-studied material YBa$_2$Cu$_3$O$_{6+x}$ (YBCO), underdoped ($p \sim 1/8$) samples show signatures of field-induced electronic and structural changes at low temperatures. However, the microscopic nature of the field-induced reconstruction and the high-field state are unclear. Here we report an x-ray study of the high-field charge density wave (CDW) in YBCO, for doping, $0.1 \lesssim p \lesssim 0.13$. For $p \sim 0.123$, we find that a field ($B \sim 10$~T) induces new CDW correlations along the CuO chain ($b$) direction only, leading to a 3-D ordered state along this direction at $B \sim 15$~T. The CDW signal along the $a$-direction is also enhanced by field, but does not develop a new pattern of correlations. We find that field modifies the coupling between the CuO$_2$ bilayers in the YBCO structure, and causes the sudden appearance of 3D CDW order. The mirror symmetry of individual bilayers is broken by the CDW at low and high fields, allowing recently suggested Fermi surface reconstruction., Comment: 4 pages 5 figures

We present a resonant inelastic x-ray scattering (RIXS) study of spin and charge excitations in overdoped La1.77Sr0.23CuO4 along two high-symmetry directions. The line shape of these excitations is analyzed and they are shown to be highly overdamped. Their spectral weight and damping are found to be strongly momentum dependent. Qualitative agreement between these observations and a calculated random-phase approximation susceptibility is obtained for this overdoped compound, implying that a significant contribution to the RIXS signal stems from a continuum of charge excitations. Furthermore, this suggests that the spin excitations in the overdoped regime can be captured qualitatively by an itinerant picture. Our calculations also predict a low-energy spin-excitation branch to exist along the nodal direction near the zone center. With the energy resolution of the present experiment, this branch is not resolvable, but we show that the next generation of high-resolution spectrometers will be able to test this prediction.

High-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen K edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism x-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λso=200 meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.

X-ray diffraction measurements show that the high-temperature superconductor YBa2Cu3O6.54, with ortho-II oxygen order, has charge-density-wave order in the absence of an applied magnetic field. The dominant wave vector of the charge density wave is q(CDW) = (0, 0.328(2), 0.5), with the in-plane component parallel to the b axis (chain direction). It has a similar incommensurability to that observed in ortho-VIII and ortho-III samples, which have different dopings and oxygen orderings. Our results for ortho-II contrast with recent high-field NMR measurements, which suggest a commensurate wave vector along the a axis. We discuss the relationship between spin and charge correlations in YBa2Cu3Oy and recent high-field quantum oscillation, NMR, and ultrasound experiments. DOI: 10.1103/PhysRevLett.110.137004

We present angle-resolved photoemission spectroscopy data on moderately underdoped La1.855Sr0.145CuO4 at temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple d-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample La1.895Sr0.105CuO4 also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV.

We study the electronic structures of two single layer superconducting cuprates, Tl$_2$Ba$_2$CuO$_{6+\delta}$ (Tl2201) and (Bi$_{1.35}$Pb$_{0.85}$)(Sr$_{1.47}$La$_{0.38}$)CuO$_{6+\delta}$ (Bi2201) which have very different maximum critical temperatures (90K and 35K respectively) using Angular Resolved Photoemission Spectroscopy (ARPES). We are able to identify two main differences in their electronic properties. First, the shadow band that is present in double layer and low T$_{c,max}$ single layer cuprates is absent in Tl2201. Recent studies have linked the shadow band to structural distortions in the lattice and the absence of these in Tl2201 may be a contributing factor in its T$_{c,max}$.Second, Tl2201's Fermi surface (FS) contains long straight parallel regions near the anti-node, while in Bi2201 the anti-nodal region is much more rounded. Since the size of the superconducting gap is largest in the anti-nodal region, differences in the band dispersion at the anti-node may play a significant role in the pairing and therefore affect the maximum transition temperature., Comment: 6 pages, 5 figures,1 table

The existence of coherent quasiparticles near the Fermi energy in the low temperature state of high-temperature superconductors has been well established by angle-resolved photoemission spectroscopy (ARPES). This technique directly probes the momentum-resolved electronic excitation spectrum of the CuO$_2$ planes. We present a study of close to optimally doped La$_{1.83}$Sr$_{0.17}$CuO$_4$ in the superconducting state and report an abrupt change in the quasiparticle spectral function, as we follow the dispersion of the ARPES signal from the Fermi energy up to 0.6 eV. The interruption in the quasiparticle dispersion separates coherent quasiparticle peaks at low energies from broad incoherent excitations at high energies. We find that the boundary between these low-energy and high-energy features exhibits a cosine-shaped momentum dependence, reminiscent of the superconducting d-wave gap. Further intriguing similarities between characteristics of the incoherent excitations and quasiparticle properties (lifetime, Fermi arcs) suggest a close relation between the electronic response at high and low energies in cuprate superconductors.

A combined resistivity and hard x-ray diffraction study of superconductivity and charge ordering in Ir Ir1−xPtxTe2, as a function of Pt substitution and externally applied hydrostatic pressure, is presented. Experiments are focused on samples near the critical composition x c ~ 0.045 where competition and switching between charge order and superconductivity is established. We show that charge order as a function of pressure in Ir0.95Pt0.05Te2 is preempted — and hence triggered — by a structural transition. Charge ordering appears uniaxially along the short crystallographic (1, 0, 1) domain axis with a (1/5, 0, 1/5) modulation. Based on these results we draw a charge-order phase diagram and discuss the relation between stripe ordering and superconductivity.