arXiv subjects

I. Bozovic

Publications and source records attributed to I. Bozovic.

At least 19 recordsLinked to original sources

arXiv2026

Enhanced and robust superconductivity in La0.8Sr0.2NiO2 membranes compressed up to 210 GPa

The discovery of superconductivity in infinite-layer nickelate thin films has opened a new frontier for exploring unconventional oxide superconductors beyond the copper oxide family. However, the presence of substrate hampers investigations under very high pressure. Recently, this problem was circumvented by a study of freestanding Nd0.85Sr0.15NiO2 membranes, which revealed that superconducting transition temperature kept increasing as the pressure was ramped up to 91 GPa, without showing signs of saturation. Naturally, one wonders what would happen at even higher pressures. Here, we report that superconductivity in a freestanding La0.8Sr0.2NiO2 membrane persists under applied ultrahigh pressures up to 210 GPa. The superconducting transition onset temperature exhibits a continuous dome-like evolution, increasing from 16 K at ambient pressure to a peak of 74.5 K at 146 GPa, and then gradually decreasing to 57.4 K at 210 GPa. Such robustness of superconductivity against ultrahigh pressure has not been observed in high-Tc oxide superconductors before.

cond-mat.supr-con↗

arXiv2026

Observation of correlated plasmons in low-valence nickelates

The discovery of nickelate superconductors has opened a new arena for studying the behavior of correlated electron liquids that give rise to unconventional superconductivity. While critical information about a material's charge dynamics is encoded in its plasmons, collective modes of the electron gas, these excitations have not yet been observed in nickelate materials. Here, we use resonant inelastic x-ray scattering (RIXS) to detect plasmons in the metallic, low-valence nickelate Pr4Ni3O8. Although qualitatively similar to those in cuprates, the nickelate plasmons are more heavily damped and have a lower velocity than those in a cuprate at comparable doping, which we attribute to reduced electronic hopping and enhanced screening of the long-range Coulomb interactions. Furthermore, the plasmons in Pr4Ni3O8 soften with increasing temperature, in contrast to the cuprate, where plasmons remain at nearly fixed energy but become more strongly damped. Taken together, these results reveal a distinct charge-screening landscape in nickelates and place quantitative constraints on analogies to cuprates.

cond-mat.str-el↗

arXiv2025

ECFA Higgs, electroweak, and top Factory Study

The ECFA Higgs, electroweak, and top Factory Study ran between 2021 and 2025 as a broad effort across the experimental and theoretical particle physics communities, bringing together participants from many different proposed future collider projects. Activities across three main working groups advanced the joint development of tools and analysis techniques, fostered new considerations of detector design and optimisation, and led to a new set of studies resulting in improved projected sensitivities across a wide physics programme. This report demonstrates the significant expansion in the state-of-the-art understanding of the physics potential of future e+e- Higgs, electroweak, and top factories, and has been submitted as input to the 2025 European Strategy for Particle Physics Update.

hep-ex↗

arXiv2025

Status of the International Linear Collider

This paper is not a proposal for a CERN future project but provides information on the International Linear Collider (ILC) considered for Japan in order to facilitate the European Strategy discussion in a global context. It describes progress to date, ongoing engineering studies, updated cost estimate for the machine at $\sqrt{s}=250~\rm GeV$ and the situation in Japan. The physics of the ILC is not presented here, but jointly for all Linear Collider projects in a separate document ``A Linear Collider Vision for the Future of Particle Physics'' submitted for the forthcoming European Strategy deliberations.

hep-ex↗

arXiv2025

The Linear Collider Facility (LCF) at CERN

In this paper we outline a proposal for a Linear Collider Facility as the next flagship project for CERN. It offers the opportunity for a timely, cost-effective and staged construction of a new collider that will be able to comprehensively map the Higgs boson's properties, including the Higgs field potential, thanks to a large span in centre-of-mass energies and polarised beams. A comprehensive programme to study the Higgs boson and its closest relatives with high precision requires data at centre-of-mass energies from the Z pole to at least 1 TeV. It should include measurements of the Higgs boson in both major production mechanisms, ee -> ZH and ee -> vvH, precision measurements of gauge boson interactions as well as of the W boson, Higgs boson and top-quark masses, measurement of the top-quark Yukawa coupling through ee ->ttH, measurement of the Higgs boson self-coupling through HH production, and precision measurements of the electroweak couplings of the top quark. In addition, ee collisions offer discovery potential for new particles complementary to HL-LHC.

hep-ex↗

arXiv2025

A Linear Collider Vision for the Future of Particle Physics

In this paper we review the physics opportunities at linear $e^+e^-$ colliders with a special focus on high centre-of-mass energies and beam polarisation, take a fresh look at the various accelerator technologies available or under development and, for the first time, discuss how a facility first equipped with a technology mature today could be upgraded with technologies of tomorrow to reach much higher energies and/or luminosities. In addition, we will discuss detectors and alternative collider modes, as well as opportunities for beyond-collider experiments and R\&D facilities as part of a linear collider facility (LCF). The material of this paper will support all plans for $e^+e^-$ linear colliders and additional opportunities they offer, independently of technology choice or proposed site, as well as R\&D for advanced accelerator technologies. This joint perspective on the physics goals, early technologies and upgrade strategies has been developed by the LCVision team based on an initial discussion at LCWS2024 in Tokyo and a follow-up at the LCVision Community Event at CERN in January 2025. It heavily builds on decades of achievements of the global linear collider community, in particular in the context of CLIC and ILC.

hep-ex↗

arXiv2022

Strongly-overdoped La$_{2-x}$Sr$_x$CuO$_4$: Evidence for Josephson-coupled grains of strongly-correlated superconductor

The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) with $x\ge0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at $T_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at $T_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at $T_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to $T_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general.

cond-mat.supr-con↗

arXiv2022

Role of Oxygen States in the Low Valence Nickelate La$_4$Ni$_3$O$_8$

The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O $K$-edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate La$_{4}$Ni$_{3}$O$_{8}$ and a prototypical cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x=0.35$). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer/Mott-Hubbard character with the Coulomb repulsion $U$ of similar size to the charge-transfer energy $Δ$. Nevertheless, the transition-metal-oxygen hopping is larger in La$_{4}$Ni$_{3}$O$_{8}$ than in La$_{2-x}$Sr$_{x}$CuO$_{4}$, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La$_{4}$Ni$_{3}$O$_{8}$ despite its larger $Δ$. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials.

cond-mat.supr-con↗

arXiv2020

Surface Josephson plasma waves in a high-temperature superconductor

Electron density oscillations with acoustic dispersions and sustained at boundaries between different media provide information about surface and interface properties of hetero-structures. In ultra-thin metallic films these plasmonic excitations are heavily damped. Superconductivity is predicted to reduce dissipation allowing detection of these resonances. Emerging low-loss interface Cooper-pair waves have been studied before, however, the observation of surface-confined Josephson plasmons has remained elusive. Here, we report on generation and coupling to these excitations in an ultrathin single-crystal film of high-temperature superconductor La1.85Sr0.15CuO4. The film becomes brighter than Au below the critical temperature when probed with sub-gap THz photons. We show that the enhanced signal in the superconducting state, which can be visualized with a spatial resolution better than λ/3,000, originates from near-field coupling of light to surface Josephson plasmons. Our results open a path towards non-invasive investigation of enhanced superconductivity in artificial multilayers, buried interface states in topological hetero-structures, and non-linear phenomena in Josephson devices.

cond-mat.supr-con↗

arXiv2020

Observation of cyclotron resonance and measurement of the hole mass in optimally-doped La$_{2-x}$Sr$_{x}$CuO$_4$

Using time-domain terahertz spectroscopy in pulsed magnetic fields up to 31 T, we measure the terahertz optical conductivity in an optimally-doped thin film of the high temperature superconducting cuprate La$_{1.84}$Sr$_{0.16}$CuO$_4$. We observe systematic changes in the circularly-polarized complex optical conductivity that are consistent with cyclotron absorption of p-type charge carriers characterized by a cyclotron mass of $4.9\pm 0.8$ $m_{\rm e}$, and a scattering rate that increases with magnetic field. These results open the door to studies aimed at characterizing the degree to which electron-electron interactions influence carrier masses in cuprate superconductors.

cond-mat.supr-con↗

arXiv2018

Scale-invariant magnetoresistance in a cuprate superconductor

The anomalous metallic state in high-temperature superconducting cuprates is masked by the onset of superconductivity near a quantum critical point. Use of high magnetic fields to suppress superconductivity has enabled a detailed study of the ground state in these systems. Yet, the direct effect of strong magnetic fields on the metallic behavior at low temperatures is poorly understood, especially near critical doping, $x=0.19$. Here we report a high-field magnetoresistance study of thin films of \LSCO cuprates in close vicinity to critical doping, $0.161\leq x\leq0.190$. We find that the metallic state exposed by suppressing superconductivity is characterized by a magnetoresistance that is linear in magnetic field up to the highest measured fields of $80$T. The slope of the linear-in-field resistivity is temperature-independent at very high fields. It mirrors the magnitude and doping evolution of the linear-in-temperature resistivity that has been ascribed to Planckian dissipation near a quantum critical point. This establishes true scale-invariant conductivity as the signature of the strange metal state in the high-temperature superconducting cuprates.

cond-mat.str-el↗

arXiv2013

Persistence of magnetic excitations in La2-xSrxCuO4 from the undoped insulator to the heavily overdoped non-superconducting metal

One of the most intensely studied scenarios of high-temperature superconductivity (HTS) postulates pairing by exchange of magnetic excitations [1]. Indeed, such excitations have been observed up to around optimal doping in the cuprates [2-7]. In the heavily overdoped regime neutron scattering measurements indicate that magnetic excitations have effectively disappeared [8-10], and this was argued to cause the demise of HTS with overdoping [1, 8, 10]. Here we use resonant inelastic x-ray scattering (RIXS), which is sensitive to complementary parts of reciprocal space, to measure the evolution of the magnetic excitations in La2-xSrxCuO4 across the entire phase diagram, from a strongly correlated insulator (x = 0) to a non-superconducting metal (x = 0.40). For x = 0, well-defined magnon excitations are observed [11]. These magnons broaden with doping, but they persist with a similar dispersion and comparable intensity all the way to the non-superconducting, heavily overdoped metallic phase. The destruction of HTS with overdoping is therefore caused neither by the general disappearance nor by the overall softening of magnetic excitations; other factor(s), such as the redistribution of spectral weight, must be considered.

cond-mat.supr-con↗

arXiv2013

Magnetic phase diagram of low-doped La{2-x}SrxCuO4 thin films studied by low-energy muon-spin rotation

The magnetic phase diagram of La2-xSrxCuO4 thin-films grown on single-crystal LaSrAlO4 substrates has been determined by low-energy muon-spin rotation. The diagram shows the same features as the one of bulk La2-xSrxCuO4, but the transition temperatures between distinct magnetic states are significantly different. In the antiferromagnetic phase the Neel temperature TN is strongly reduced, and no hole spin freezing is observed at low temperatures. In the disordered magnetic phase (x>0.02) the transition temperature to the cluster spin-glass state Tg is enhanced. Possible reasons for the pronounced differences between the magnetic phase diagrams of thin-film and bulk samples are discussed.

cond-mat.mtrl-sci↗

arXiv2012

Spin excitations in a single La$_2$CuO$_4$ layer

The dynamics of S=1/2 quantum spins on a 2D square lattice lie at the heart of the mystery of the cuprates \cite{Hayden2004,Vignolle2007,Li2010,LeTacon2011,Coldea2001,Headings2010,Braicovich2010}. In bulk cuprates such as \LCO{}, the presence of a weak interlayer coupling stabilizes 3D Néel order up to high temperatures. In a truly 2D system however, thermal spin fluctuations melt long range order at any finite temperature \cite{Mermin1966}. Further, quantum spin fluctuations transfer magnetic spectral weight out of a well-defined magnon excitation into a magnetic continuum, the nature of which remains controversial \cite{Sandvik2001,Ho2001,Christensen2007,Headings2010}. Here, we measure the spin response of \emph{isolated one-unit-cell thick layers} of \LCO{}. We show that coherent magnons persist even in a single layer of \LCO{} despite the loss of magnetic order, with no evidence for resonating valence bond (RVB)-like spin correlations \cite{Anderson1987,Hsu1990,Christensen2007}. Thus these excitations are well described by linear spin wave theory (LSWT). We also observe a high-energy magnetic continuum in the isotropic magnetic response. This high-energy continuum is not well described by 2 magnon LSWT, or indeed any existing theories.

cond-mat.str-el↗

arXiv2011

Graded Orbital Occupation near Interfaces in a La2NiO4 - La2CuO4 Superlattice

X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a non-uniform distribution of oxygen holes in a La2NiO4 - La2CuO4 (LNO-LCO) superlattice, with excess holes concentrated in the LNO layers. Weak ferromagnetism with Tc = 160 K suggests a coordinated tilting of NiO6 octahedra, similar to that of bulk LNO. Ni d3z2-r2 orbitals within the LNO layers have a spatially variable occupation. This variation of the Ni valence near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at the Ni L edge, at a reflection suppressed by the symmetry of the structure, and is possible through graded doping with holes, due to oxygen interstitials taken up preferentially by inner LNO layers. Since the density of oxygen atoms in the structure can be smoothly varied with standard procedures, this orbital occupation, robust up to at least 280 K, is tunable.

cond-mat.str-el↗

arXiv2011

Distinct oxygen hole doping in different layers of $\rm Sr_{2}CuO_{4-δ}-La_{2}CuO_4$ superlattices

X-ray absorption in $\rm Sr_{2}CuO_{4-δ}-La_{2}CuO_4$ (SCO-LCO) superlattices shows a variable occupation with doping of a hole state different from holes doped for $x \lesssim x_{optimal}$ in bulk $\rm La_{2-x}Sr_{x}CuO_4$ and suggests that this hole state is on apical oxygen atoms and polarized in the $a-b$ plane. Considering the surface reflectivity gives a good qualitative description of the line shapes of resonant soft X-ray scattering. The interference between superlattice and surface reflections was used to distinguish between scatterers in the SCO and the LCO layers, with the two hole states maximized in different layers of the superlattice.

cond-mat.supr-con↗

arXiv2011

Superconductivity in La(1.56)Sr(0.44)CuO(4)/La(2)CuO(4) superlattices

Superlattices of the repeated structure La(1.56)Sr(0.44)CuO(4)/La(2)CuO(4) (LSCO-LCO), where none of the constituents is superconducting, show a superconducting transition of T_c \simeq 25 K. In order to elucidate the nature of the superconducting state we have performed a low-energy muSR study. By applying a magnetic field parallel (Meissner state) and perpendicular (vortex state) to the film planes, we could show that superconductivity is sheet like, resulting in a very anisotropic superconducting state. This result is consistent with a simple charge-transfer model, which takes into account the layered structure and the difference in the chemical potential between LCO and LSCO, as well as Sr interdiffusion. Using a pancake-vortex model we could estimate a strict upper limit of the London penetration depth to 380 nm in these superlattices. The temperature dependence of the muon depolarization rate in field cooling experiments is very similar to what is observed in intercalated BSCCO and suggests that vortex-vortex interaction is dominated by electromagnetic coupling but negligible Josephson interaction.

cond-mat.supr-con↗

arXiv2011

Magnetism in the 2D Limit and Interface Superconductivity in Metal-Insulator La(2-x)Sr(x)CuO(4) Superlattices

We show, by means of low-energy muon spin rotation measurements, that few-unit-cells thick La(2)CuO(4) layers synthesized digitally by molecular beam epitaxy synthesis are antiferromagnetically ordered. Below a thickness of about 5 CuO(2) layers the long-range ordered state breaks down, and a magnetic state appears with enhanced quantum fluctuations and a reduced spin stiffness. This magnetic state can exist in close proximity (few Angstrom) to high-temperature superconducting layers, without transmitting supercurrents.

cond-mat.str-el↗