arXiv subjects

M. Greven

Publications and source records attributed to M. Greven.

At least 19 recordsLinked to original sources

arXiv2026

Dynamic nanoscale structural correlations in strontium ruthenate

Strontium ruthenate (Sr$_2$RuO$_4$, SRO) has been the subject of intense research as a model quasi-two-dimensional metal with strong electronic correlations and potential exotic multi-component superconductor. Yet the nature of the superconducting state and its emergence remain debated, despite highly detailed knowledge of the normal-state electronic properties. Here we use diffuse neutron and x-ray scattering to uncover self-organized structural fluctuations on the picosecond timescale in SRO. We show that these nanoscale correlations do not originate from extrinsic disorder but rather involve cooperative displacements of oxygen atoms in the quintessential RuO$_2$ planes. Moreover, the observed displacement pattern is consistent with distortions due to incipient orbital order that we obtain in first-principles calculations, which suggests that orbital effects could play a pivotal role in the physics of SRO. Similar dynamic correlations may play a role in the physical properties of a wide range of prominent oxides with closely-related lamellar structures, such as the cuprates and nickelates.

cond-mat.str-el↗

arXiv2026

Spin-chiral electron-phonon coupling in metallic strontium titanate

Electron-phonon coupling (EPC) - the interaction between conduction electrons and quantized atomic vibrations - plays a central role in condensed matter physics and determines some of the most important properties of materials, such as electrical resistivity and superconductivity. Conventionally, EPC is assumed to be induced by the ionic electrostatic background, and electronic spin plays no role in the process. In stark contrast with this view, here we uncover a direct spin-mediated coupling mechanism between electrons and transverse polar phonons in a metal. Using far-infrared light absorption measurements of the model system SrTiO$_3$ in a magnetic field, we observe a strong spin-mediated EPC that is quantitatively consistent with recent theoretical predictions, and that generates chiral phonon modes with large effective magnetic moments. The extracted coupling strength is in good agreement with ab initio estimates and sufficiently high to explain superconductivity in SrTiO$_3$, thereby resolving a long-standing conundrum. Spin-chiral EPC should generically appear in all metals with polar phonons, and the present work could be of relevance to spintronics applications and to uncovering the origins of superconductivity in layered materials, metals with Dirac points in their electronic dispersions, and nearly ferroelectric superconductors.

cond-mat.str-el↗

arXiv2025

Evidence for rare-region physics in the structural and electronic degrees of freedom of the nickelate La$_{2-x}$Sr$_x$NiO$_4$

We present a diffuse neutron and x-ray scattering study of structural as well as spin- and charge-density-wave fluctuations in the electrical insulator La$_{2-x}$Sr$_x$NiO$_4$. This lamellar nickelate is an isostructural analogue of the high-temperature cuprate superconductor La$_{2-x}$Sr$_x$CuO$_4$, for which recent experiments uncovered evidence for unusual structural and superconducting fluctuations indicative of rare-region physics due to inherent inhomogeneity unrelated to common point disorder effects. We find closely analogous nanoscale orthorhombic fluctuation behavior in La$_{2-x}$Sr$_x$NiO$_4$, including exponential scaling of the diffuse scattering intensity and power-law scaling of the characteristic length with relative temperature. Moreover, our neutron and x-ray scattering data reveal similar behavior for short-range magnetic and charge fluctuations above the respective ordering temperatures. These observations indicate that rare-region effects are a generic feature of perovskite-related structures and lead to universal fluctuations of both structural and electronic degrees of freedom over extended temperature ranges.

cond-mat.str-el↗

arXiv2025

Conventional superconductivity in single-crystalline BiPt

Binary Bi-Pd/Pt systems have attracted a lot of interest because of their topologically non-trivial nature along with superconductivity. We report the structural and superconducting properties of high-quality single-crystalline BiPt using a comprehensive range of experimental techniques, including X-ray diffraction, electron microscopy, muon spin rotation/relaxation ({\mu}SR), magnetization, resistivity, and heat capacity. Our findings establish that BiPt is a weak type-II superconductor with a transition temperature (Tc) of 1.2 K which exhibits pronounced anisotropic superconducting characteristics attributed to its hexagonal crystal structure. Magnetization and electronic transport studies reveal that BiPt lies within the dirty limit, while {\mu}SR and heat capacity data indicate conventional s-wave superconductivity that maintains time-reversal symmetry. This work provides valuable insights into the pairing symmetry and superconducting mechanism of topologically trivial BiPt, a sound comparison system for other Bi-based topologically nontrivial superconductors.

cond-mat.supr-con↗

arXiv2025

Uniaxial strain tuning of polar lattice vibrations in KTaO$_3$ and SrTiO$_3$

The interplay of electronic and structural degrees of freedom is a prominent feature of many quantum materials and of particular interest in systems with strong ferroelectric fluctuations, such as SrTiO$_3$ (STO) and KTaO$_3$ (KTO). Both materials are close to a ferroelectric transition, but despite six decades of extensive research, pivotal questions regarding the nature of this transition and of the associated fluctuations remain debated. Here we combine inelastic neutron scattering, Raman spectroscopy, and ab initio calculations to study the evolution of soft polar phonons across the strain-induced ferroelectric transition in STO and KTO. We find that the modes remain underdamped and at nonzero energy, consistent with a first-order quantum phase transition. We also reveal a strong violation of the well-known Lyddane-Sachs-Teller relation between the phonon energies and static dielectric permittivities in insulating KTO and STO, which is not captured by ab initio calculations and points to the presence of slow mesoscale fluctuations induced by long-range interactions. In metallic STO, we uncover a first-order transition at a remarkably low critical stress, in qualitative agreement with recent theoretical predictions. The present work resolves several long-standing questions pertaining to the model systems STO and KTO and is relevant to numerous other materials with soft polar phonons.

cond-mat.mtrl-sci↗

arXiv2025

Nature of the ferromagnet-paramagnet transition in Y$_{1-x}$Ca$_{x}$TiO$_{3}$

Neutron scattering, magnetometry, and muon spin rotation ($\mu$SR) measurements were performed to investigate the magnetic order and spin dynamics across the ferromagnet-to-paramagnet transition in the hole-doped Mott insulator Y$_{1-x}$Ca$_x$TiO$_3$. We find that the transition proceeds through a volume-wise phase separation into ferromagnetic and paramagnetic regions. Spin fluctuations with a characteristic timescale of $\sim$ 0.1 $\mu$s, as detected via $\mu$SR, are observed to appear at Ca concentrations $x \geq 0.10$. The magnetic phase separation, accompanied by a modest dynamic response, represents a novel behavior in Mott systems near the loss of magnetic order. It is linked to a previously observed insulator-metal transition and the associated electronic phase separation into hole-poor Mott insulating and hole-rich metallic phases for $0 < x < 0.50$. In particular, the $x$-dependence of the paramagnetic volume fraction strongly correlates with that of the volume fraction of the hole-rich metallic phase. The spin-wave spectra reveal a doping-induced crossover from isotropic to two-dimensional anisotropic exchange interactions, reflecting substantial changes in the orbital state with increasing Ca content.

cond-mat.str-el↗

arXiv2025

Pervasive symmetry-lowering nanoscale structural fluctuations in the cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$

The cuprate superconductors are among the most widely studied quantum materials, yet there remain fundamental open questions regarding their electronic properties and the role of the structural degrees of freedom. Recent neutron and x-ray scattering measurements uncovered exponential scaling with temperature of the strength of orthorhombic fluctuations in the tetragonal phase of $La_{2-x}Sr_xCuO_4$ and $Tl_2Ba_2CuO_{6+y}$, unusual behavior that closely resembles prior results for the emergence of superconducting fluctuations, and that points to a common origin rooted in inherent correlated structural inhomogeneity. Here we extend the measurements of $La_{2-x}Sr_xCuO_4$ to higher temperatures in the parent compound (x=0) and to optimal doping (x=0.155), and we furthermore investigate the effects of in-situ in-plane uniaxial stress. Our neutron scattering result for undoped $La_2CuO_4$ complement prior x-ray data and reveal that the structural fluctuations persist to the maximum experimental temperature of nearly 1000K, i.e., to a significant fraction of the melting point. At this temperature, the spatial correlation length extracted from the momentum-space data is still about three lattice constants. The neutron scattering experiment enables quasistatic discrimination and reveals that the response is increasingly dynamic at higher temperatures. We also find that uniaxial stress up to 500 MPa along the tetragonal [110] direction, which corresponds to a strain of about 0.2%, does not significantly alter this robust behavior. Overall, these results support the notion that subtle, underlying inhomogeneity underpins the cuprate phase diagram. Finally, we uncover (for x=0.2) low-energy structural fluctuations at a nominally forbidden reflection. While the origin of these fluctuations is not clear, they might be related to the presence of extended defects such as dislocations or stacking faults.

cond-mat.supr-con↗

arXiv2024

Electronic spin susceptibility in metallic strontium titanate

Metallic strontium titanate (SrTiO$_3$) is known to have both normal-state and superconducting properties that vary strongly over a wide range of charge carrier densities. This indicates the importance of nonlinear dynamics, and has hindered the development of a clear qualitative description of the observed behaviour. A major challenge is to understand how the charge carriers themselves evolve with doping and temperature, with possible polaronic effects and evidence of an effective mass that strongly increases with temperature. Here we use $^{47,49}$Ti nuclear magnetic resonance (NMR) to perform a comprehensive study of the electronic spin susceptibility in the dilute metallic state of strontium titanate across the doping-temperature phase diagram. We find a temperature-dependent Knight shift that can be quantitatively understood within a non-degenerate Fermi gas model that fully takes into account the complex band structure of SrTiO$_3$. Our data are consistent with a temperature-independent effective mass, and we show that the behavior of the spin susceptibility is universal in a wide range of temperatures and carrier concentrations. These results provide a microscopic foundation for the understanding of the properties of the unconventional low-density metallic state in strontium titanate and related materials.

cond-mat.str-el↗

arXiv2023

Effect of random antiferromagnetic exchange on the spin waves in a three-dimensional Heisenberg ferromagnet

Neutron scattering is used to study spin waves in the three-dimensional Heisenberg ferromagnet YTiO$_3$, with spin-spin exchange disorder introduced $via$ La-substitution at the Y site. No significant changes are observed in the spin-wave dispersion up to a La concentration of 20%. However, a strong broadening of the spectrum is found, indicative of shortened spin-wave lifetimes. Density-functional theory calculations predict minimal changes in exchange constants as a result of average structural changes due to La substitution, in agreement with the data. The absence of significant changes in the spin-wave dispersion, the considerable lifetime effect, and the reduced ordered magnetic moment previously observed in the La-substituted system are qualitatively captured by an isotropic, nearest-neighbor, three-dimensional Heisenberg ferromagnet model with random antiferromagnetic exchange. We therefore establish Y$_{1-x}$La$_x$TiO$_3$ as a model system to study the effect of antiferromagnetic spin-exchange disorder in a three-dimensional Heisenberg ferromagnet.

cond-mat.str-el↗

arXiv2022

Local inversion-symmetry breaking in a bismuthate high-$T_c$ superconductor

The doped perovskite BaBiO$_3$ exhibits a maximum superconducting transition temperature ($T_c$) of 34 K and was the first high-$T_c$ oxide to be discovered, yet pivotal questions regarding the nature of both the metallic and superconducting states remain unresolved. Although it is generally thought that superconductivity in the bismuthates is of the conventional s-wave type, the pairing mechanism is still debated, with strong electron-phonon coupling and bismuth valence or bond disproportionation possibly playing a role. Here we use diffuse x-ray scattering and Monte Carlo modeling to study the local structure of Ba$_{1-x}$K$_x$BiO$_3$ across its insulator-metal boundary. We find no evidence for either long- or short-range disproportionation, which resolves a major conundrum, as disproportionation and the related polaronic effects are likely not relevant for the metallic and superconducting states. Instead, we uncover nanoscale structural correlations that break inversion symmetry, which has far-reaching implications for the electronic physics, including the pairing mechanism. This unexpected finding furthermore establishes that the bismuthates belong to the broader classes of materials with hidden spin-orbit coupling and a tendency towards inversion-breaking displacements.

cond-mat.supr-con↗

arXiv2021

Growth and characterization of large (Y,La)TiO$_3$ and (Y,Ca)TiO$_3$ single crystals

The Mott-insulating rare-earth titanates (RTiO$_3$, R being a rare-earth ion) are an important class of materials that encompasses interesting spin-orbital phases as well as ferromagnet-antiferromagnet and insulator-metal transitions. The growth of these materials has been plagued by difficulties related to overoxidation, which arises from a strong tendency of Ti$^{3+}$ to oxidize to Ti$^{4+}$. We describe our efforts to grow sizable single crystals of YTiO$_3$ and its La-substituted and Ca-doped variants with the optical travelling-solvent floating-zone technique. We present sample characterization $via$ chemical composition analysis, magnetometry, charge transport, neutron scattering, x-ray absorption spectroscopy and x-ray magnetic circular dichroism to understand macroscopic physical property variations associated with overoxidation. Furthermore, we demonstrate a good signal-to-noise ratio in inelastic magnetic neutron scattering measurements of spin-wave excitations. A superconducting impurity phase, found to appear in Ca-doped samples at high doping levels, is identified as TiO.

cond-mat.str-el↗

arXiv2021

Arc-to-pocket transition and quantitative understanding of transport properties in cuprate superconductors

Despite immense efforts, the cuprate Fermi surface (FS) has been unambiguously determined in only two distinct, low-temperature regions of the phase diagram: a large hole-like FS at high doping, and a small electron-like pocket associated with charge-density-wave driven FS reconstruction at moderate doping. Moreover, there exists incomplete understanding of the reconstructed state, which is stabilized by high magnetic fields, and its connection with the normal state that consists of arc-like remnants of the large underlying FS. Part of the problem is that compound-specific idiosyncrasies, such as disorder effects and low structural symmetry, can obscure the fundamental properties of the quintessential CuO$_2$ planes. Here we present planar magnetotransport measurements for moderately-doped HgBa$_2$CuO$_{4+{\delta}}$ that enable a quantitative understanding of the phase transition between the normal and reconstructed states and of the charge transport in the latter, and that demonstrate that the quasiparticle scattering rate in both states is due to Umklapp scattering. Building on prior insights, we furthermore arrive at a comprehensive understanding of the evolution of the planar transport properties throughout the entire cuprate phase diagram.

cond-mat.supr-con↗

arXiv2021

Nature of the ferromagnetic-antiferromagnetic transition in Y$_{1-x}$La$_{x}$TiO$_{3}$

We explore the magnetically-ordered ground state of the isovalently-substituted Mott-insulator Y$_{1-x}$La$_{x}$TiO$_{3}$ for $x$ $\leq$ 0.3 via single crystal growth, magnetometry, neutron diffraction, x-ray magnetic circular dichroism (XMCD), muon spin rotation ($\mu$SR) and small-angle neutron scattering (SANS). We find that the decrease in the magnetic transition temperature on approaching the ferromagnetic (FM) - antiferromagnetic (AFM) phase boundary at the La concentration $x_c$ $\approx$ 0.3 is accompanied by a strong suppression of both bulk and local ordered magnetic moments, along with a volume-wise separation into magnetically-ordered and paramagnetic regions. The thermal phase transition does not show conventional second-order behavior, since neither a clear signature of dynamic critical behavior nor a power-law divergence of the magnetic correlation length is found for the studied substitution range; this finding becomes increasingly obvious with substitution. Finally, from SANS and magnetometry measurements, we discern a crossover from easy-axis to easy-plane magneto-crystalline anisotropy with increasing La substitution. These results indicate complex changes in magnetic structure upon approaching the phase boundary.

cond-mat.str-el↗

arXiv2021

Two-component electronic phase separation in the doped Mott insulator Y$_{1-x}$Ca$_{x}$TiO$_{3}$

One of the major puzzles in condensed matter physics has been the observation of a Mott-insulating state away from half-filling. The filling-controlled Mott insulator-metal transition, induced via charge-carrier doping, has been extensively researched, but its governing mechanisms have yet to be fully understood. Several theoretical proposals aimed to elucidate the nature of the transition have been put forth, a notable one being phase separation and an associated percolation-induced transition. In the present work, we study the prototypical doped Mott-insulating rare-earth titanate YTiO$_3$, in which the insulating state survives up to a large hole concentration of 35%. Single crystals of Y$_{1-x}$Ca$_x$TiO$_3$ with $0 \leq x \leq 0.5$, spanning the insulator-metal transition, are grown and investigated. Using x-ray absorption spectroscopy, a powerful technique capable of probing element-specific electronic states, we find that the primary effect of hole doping is to induce electronic phase separation into hole-rich and hole-poor regions. The data reveal the formation of electronic states within the Mott-Hubbard gap, near the Fermi level, which increase in spectral weight with increasing doping. From a comparison with DFT+$U$ calculations, we infer that the hole-poor and hole-rich components have charge densities that correspond to the Mott-insulating $x = 0$ and metallic $x \sim 0.5$ states, respectively, and that the new electronic states arise from the metallic component. Our results indicate that the hole-doping-induced insulator-metal transition in Y$_{1-x}$Ca$_x$TiO$_3$ is indeed percolative in nature, and thus of inherent first-order character.

cond-mat.str-el↗

arXiv2021

Unconventional short-range structural fluctuations in cuprate high-$T_c$ superconductors

The interplay between structural and electronic degrees of freedom in complex materials is the subject of extensive debate in physics and materials science. Particularly interesting questions pertain to the nature and extent of pre-transitional short-range order in diverse systems ranging from shape-memory alloys to unconventional superconductors, and how this microstructure affects macroscopic properties. Here we use neutron and X-ray diffuse scattering to uncover universal structural fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ and Tl$_2$Ba$_2$CuO$_{6+\delta}$, two cuprate superconductors with distinct point disorder effects and optimal superconducting transition temperatures. The fluctuations are present in wide doping and temperature ranges, including compositions that maintain high average structural symmetry, and they exhibit unusual, yet simple scaling behavior. The scaling regime is robust and universal, similar to the well-known critical fluctuations close to second-order phase transitions, but with a distinctly different physical origin. We relate this behavior to pre-transitional phenomena in a broad class of systems with structural and magnetic transitions, and propose an explanation based on rare structural fluctuations caused by intrinsic nanoscale inhomogeneity. We also uncover parallels with superconducting fluctuations, which indicates that the underlying inhomogeneity plays an important role in cuprate physics.

cond-mat.supr-con↗

arXiv2020

Anomalous mirror symmetry breaking in a model insulating cuprate Sr$_2$CuO$_2$Cl$_2$

Understanding the complex phase diagram of cuprate superconductors is an outstanding challenge. The most actively studied questions surround the nature of the pseudogap and strange metal states and their relationship to superconductivity. In contrast, there is general agreement that the low energy physics of the Mott insulating parent state is well captured by a two-dimensional spin $S$ = 1/2 antiferromagnetic (AFM) Heisenberg model. However, recent observations of a large thermal Hall conductivity in several parent cuprates appear to defy this simple model and suggest proximity to a magneto-chiral state that breaks all mirror planes perpendicular to the CuO$_2$ layers. Here we use optical second harmonic generation to directly resolve the point group symmetries of the model parent cuprate Sr$_2$CuO$_2$Cl$_2$. We report evidence of an order parameter $\Phi$ that breaks all perpendicular mirror planes and is consistent with a magneto-chiral state in zero magnetic field. Although $\Phi$ is clearly coupled to the AFM order parameter, we are unable to realize its time-reversed partner ($-\Phi$) by thermal cycling through the AFM transition temperature ($T_{\textrm{N}}$ $\approx$ 260 K) or by sampling different spatial locations. This suggests that $\Phi$ onsets above $T_{\textrm{N}}$ and may be relevant to the mechanism of pseudogap formation.

cond-mat.str-el↗

arXiv2020

Three interaction energy scales in single-layer high-T$_C$ cuprate HgBa$_2$CuO$_{4+\delta}$

The lamellar cuprate superconductors exhibit the highest ambient-pressure superconducting transition temperatures (T$_C$) and, after more than three decades of extraordinary research activity, continue to pose formidable scientific challenges. A major experimental obstacle has been to distinguish universal phenomena from materials- or technique-dependent ones. Angle-resolved photoemission spectroscopy (ARPES) measures momentum-dependent single-particle electronic excitations and has been invaluable in the endeavor to determine the anisotropic momentum-space properties of the cuprates. HgBa$_2$CuO$_{4+\delta}$ (Hg1201) is a single-CuO$_2$-layer cuprate with a particularly high optimal T$_C$ and a simple crystal structure; yet there exists little information from ARPES about the electronic properties of this model system. Here we present an ARPES study of doping-, temperature-, and momentum-dependent systematics of near-nodal dispersion anomalies in Hg1201. The data reveal a hierarchy of three distinct energy scales -a sub-gap low-energy kink, an intermediate-energy kink near 55 meV, and a peak-dip-hump structure. The first two features are attributed to the coupling of electrons to Ba-derived optical phonons and in-plane bond-stretching phonons, respectively. The nodal peak-dip-hump structure appears to have a common doping-dependence in several single-layer cuprates, and is interpreted as a manifestation of pseudogap physics at the node. These results establish several universal phenomena, both in terms of connecting multiple experimental techniques for a single material, and in terms of connecting comparable spectral features in multiple structurally similar cuprates.

cond-mat.supr-con↗

arXiv2020

Ferroelectric quantum criticality and enhanced superconductivity in plastically deformed strontium titanate

The properties of quantum materials are commonly tuned using experimental variables such as pressure, magnetic field and doping. Here we explore a different approach: irreversible, plastic deformation of single crystals. We show for the superconductor SrTiO$_3$ that compressive plastic deformation induces low-dimensional superconductivity significantly above the superconducting transition temperature ($T_c$) of undeformed samples, with evidence of superconducting correlations at temperatures two orders of magnitude above the bulk $T_c$. The superconductivity enhancement is correlated with the appearance of self-organized dislocation structures, as revealed by diffuse neutron and X-ray scattering. We also observe signatures of deformation-induced quantum-critical ferroelectric fluctuations and inhomogeneous ferroelectric order via Raman scattering. These results suggest that the strain surrounding the self-organized dislocation structures induces local ferroelectricity and quantum-critical dynamics that strongly influence $T_c$, consistent with a theory of superconductivity enhanced by soft polar fluctuations. More broadly, our results demonstrate the promise of plastic deformation and dislocation engineering as tools to manipulate electronic properties of quantum materials.

cond-mat.supr-con↗