arXiv subjects

S. Rosenkranz

Publications and source records attributed to S. Rosenkranz.

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↗

arXiv2023

Stacking disorder in $α$-RuCl$_3$ via x-ray three-dimensional difference pair distribution function analysis

The van der Waals layered magnet $α$-RuCl$_3$ offers tantalizing prospects for the realization of Majorana quasiparticles. Efforts to understand this are, however, hampered by inconsistent magnetic and thermal transport properties likely coming from the formation of structural disorder during crystal growth, postgrowth processing, or upon cooling through the first order structural transition. Here, we investigate structural disorder in $α$-RuCl$_3$ using x-ray diffuse scattering and three-dimensional difference pair distribution function (3D-$Δ$PDF) analysis. We develop a quantitative model that describes disorder in $α$-RuCl$_3$ in terms of rotational twinning and intermixing of the high and low-temperature structural layer stacking. This disorder may be important to consider when investigating the detailed magnetic and electronic properties of this widely studied material.

cond-mat.str-el↗

arXiv2023

Two-Dimensional Short-Range Chemical Ordering in Ba1-xNaxFe2As2

A true understanding of the properties of pnictide superconductors require the development of high-quality materials and performing measurements designed to unravel their intrinsic properties and short-range nematic correlations which are often obscured by extrinsic effects such as poor crystallinity, inhomogeneity, domain formation and twinning. In this paper, we report the systematic growth of high-quality Na-substituted BaFe2As2 single crystals and their characterization using pulsed-magnetic fields x-ray diffraction and x-ray diffuse scattering. Analysis of the properties and compositions of the highest quality crystals show that their actual Na stoichiometry is about 50-60% of the nominal content and that the targeted production of crystals with specific compositions is accessible. We derived a reliable equation to estimate the Na stoichiometry based on the measured superconducting TC of these materials. Attempting to force spin reorientation and induce tetragonality, orthorhombic Ba1-xNaxFe2As2 single crystals subjected to out-of-plane magnetic fields up to 31.4T are found to exhibit strong in-plane magnetic anisotropy demonstrated by the insufficiency of such high fields in manipulating the relative population of their twinned domains or in suppressing the orthorhombic order. Broad x-ray diffuse intensity rods observed at temperatures between 30 K and 300 K uncover short-range structural correlations. Local structure modeling together with 3D-ΔPDF mapping of real-space interatomic vectors show that the diffuse scattering arises from in-plane short-range chemical correlations of the Ba and Na atoms coupled with short-range atomic displacements within the same plane due to an effective size difference between the two atomic species.

cond-mat.supr-con↗

arXiv2023

Structure of Charge Density Waves in La$_{1.875}$Ba$_{0.125}$CuO$_4$

Although charge-density wave (CDW) correlations exist in several families of cuprate supercon-ductors, they exhibit substantial variation in CDW wavevector and correlation length, indicating a key role for CDW-lattice interactions. We investigated this interaction in La$_{1.875}$Ba$_{0.125}$CuO$_4$ using single crystal x-ray diffraction to collect a large number of CDW peak intensities, and determined the Cu and La/Ba atomic distortions induced by the formation of CDW order. Within the CuO$_2$ planes, the distortions involve a periodic modulation of the Cu-Cu spacing along the direction of the ordering wave vector. The charge ordering within the copper-oxygen layer induces an out-of-plane breathing modulation of the surrounding lanthanum layers, which leads to a related distortion on the adjacent copper-oxygen layer. Our result implies that the CDW-related structural distortions do not remain confined to a single layer but rather propagate an appreciable distance through the crystal. This leads to overlapping structural modulations, in which CuO$_2$ planes exhibit distortions arising from the orthogonal CDWs in adjacent layers as well as distortions from the CDW within the layer itself. We attribute this striking effect to the weak c-axis charge screening in cuprates and suggest this effect could help couple the CDW between adjacent planes in the crystal.

cond-mat.supr-con↗

arXiv2022

Bootstrapped Dimensional Crossover of a Spin Density Wave

Quantum materials display rich and myriad types of magnetic, electronic, and structural ordering, often with these ordering modes either competing with one another or 'intertwining', that is, reinforcing one another. Low dimensional quantum materials, influenced strongly by competing interactions and/or geometric frustration, are particularly susceptible to such ordering phenomena and thus offer fertile ground for understanding the consequent emergent collective quantum phenomena. Such is the case of the quasi-2D materials R4Ni3O10(R=La, Pr), in which intertwined charge-and spin-density waves (CDW and SDW) on the Ni sublattice have been identified and characterized. Not unexpectedly, these density waves are largely 2D as a result of weak coupling between planes, compounded with magnetic frustration. In the case of R=Pr, however, we show here that exchange coupling between the transition metal and rare earth sublattices upon cooling overcomes both obstacles, leading to a dimensional crossover into a fully 3D ordered and coupled SDW state on both sublattices, as an induced moment on notionally nonmagnetic Pr3+ opens exchange pathways in the third dimension. In the process, the structure of the SDW on the Ni sublattice is irreversibly altered, an effect that survives reheating of the material until the underlying CDW melts. This 'bootstrapping' mechanism linking incommensurate SDWs on the two sublattices illustrates a new member of the multitude of quantum states that low-dimensional magnets can express, driven by coupled orders and modulated by frustrated exchange pathways.

cond-mat.str-el↗

arXiv2022

Multimodal synchrotron X-ray diffraction across the superconducting transition of Sr$_{0.1}$Bi$_2$Se$_3$

In the doped topological insulator Sr$_x$Bi$_2$Se$_3$, a pronounced in-plane two-fold symmetry is observed in electronic properties below the superconducting transition temperature $T_c \sim$ 3 K, despite the three-fold symmetry of the observed $R\bar{3}m$ space group. The axis of two-fold symmetry is nominally pinned to one of three rotational equivalent directions and crystallographic strain has been proposed to be the origin of this pinning. We carried out multimodal synchrotron diffraction and resistivity measurements down to $\sim$0.68 K and in magnetic fields up to 45 kG on a single crystal of Sr$_{0.1}$Bi$_2$Se$_3$ to probe the effect of superconductivity on the crystallographic distortion. Our results indicate that there is no in-plane crystallographic distortion at the level of $1x10^{-5}$ associated with the superconducting transition. These results further support the model that the large two-fold in-plane anisotropy of superconducting properties of Sr$_x$Bi$_2$Sr$_3$ is not structural in origin but electronic, namely it is caused by a nematic superconducting order parameter of Eu symmetry.

cond-mat.supr-con↗

arXiv2022

Neutron Scattering Study of Fluctuating and Static Spin Correlations in the Anisotropic Spin Glass Fe$_2$TiO$_5$

The anisotropic spin glass transition, in which spin freezing is observed only along the c-axis in pseudobrookite Fe$_2$TiO$_5$, has long been perplexing because the Fe$^{3+}$ moments (d$^5$) are expected to be isotropic. Recently, neutron diffraction demonstrated that surfboard-shaped antiferromagnetic nanoregions coalesce above the glass transition temperature, T$_g$ $\approx$ 55 K, and a model was proposed in which the freezing of the fluctuations of the surfboards' magnetization leads to the anisotropic spin glass state. Given this new model, we have carried out high resolution inelastic neutron scattering measurements of the spin-spin correlations to understand the temperature dependence of the intra-surfboard spin dynamics on neutron (picosecond) time-scales. Here, we report on the temperature-dependence of the spin fluctuations measured from single crystal Fe$_2$TiO$_5$. Strong quasi-elastic magnetic scattering, arising from intra-surfboard correlations, is observed well above T$_g$. The spin fluctuations possess a steep energy-wave vector relation and are indicative of strong exchange interactions, consistent with the large Curie-Weiss temperature. As the temperature approaches T$_g$ from above, a shift in spectral weight from inelastic to elastic scattering is observed. At various temperatures between 4 K and 300 K, a characteristic relaxation rate of the fluctuations is determined. Despite the freezing of the majority of the spin correlations, an inelastic contribution remains even at base temperature, signifying the presence of fluctuating intra-surfboard spin correlations to at least T/T$_g$ $\approx$ 0.1 consistent with a description of Fe$_2$TiO$_5$ as a hybrid between conventional and geometrically frustrated spin glasses.

cond-mat.dis-nn↗

arXiv2022

The Geometrically Frustrated Spin Glass (Fe1-pGap)2TiO5

The unusual anisotropy of the spin glass transition in the pseudobrookite system Fe$_2$TiO$_5$ has been interpreted as arising from an induced, van der Waals-like, interaction among magnetic clusters. Here we present susceptibility ($χ$) and specific heat data (C) for Fe2TiO5 diluted with non-magnetic Ga, (Fe$_{1-p}$Ga$_p$)$_2$TiO$_5$, for disorder parameter p = 0, 0.11, and 0.42, and elastic neutron scattering data for p = 0.20. A uniform suppression of T{_g} is observed upon increasing p, along with a value of $χ(T_g)$ that increases as T$_g$ decreases, i.e. $dχ(T_g)/dT_g< 0$. We also observe C(T) $\propto$ T$^2$ in the low temperature limit. The observed behavior places (Fe$_{1-p}$Ga$_p$)$_2$TiO$_5$ in the category of a strongly geometrically frustrated spin glass.

cond-mat.dis-nn↗

arXiv2020

Fluctuation-Induced Interactions and the Spin Glass Transition in $Fe_2TiO_5$

We investigate the spin-glass transition in the strongly frustrated well-known compound $Fe_2TiO_5$. A remarkable feature of this transition, widely discussed in the literature, is its anisotropic properties: the transition manifests itself in the magnetic susceptibly only along one axis, despite $Fe^{3+}$ $d^5$ spins having no orbital component. We demonstrate, using neutron scattering, that below the transition temperature $T_g = 55 K$, $Fe_2TiO_5$ develops nanoscale surfboard shaped antiferromagnetic regions in which the $Fe^{3+}$ spins are aligned perpendicular to the axis which exhibits freezing. We show that the glass transition may result from the freezing of transverse fluctuations of the magnetization of these regions and we develop a mean-field replica theory of such a transition, revealing a type of magnetic van der Waals effect.

cond-mat.dis-nn↗

arXiv2020

Soft elastic constants from phonon spectroscopy in hole-doped Ba$_{1-x}$(K,Na)$_x$Fe$_2$As$_2$ and Sr$_{1-x}Na$_x$Fe$_2$As$_2$

We report inelastic x-ray scattering measurements of the in-plane polarized transverse acoustic phonon mode propagating along $q\parallel$[100] in various hole-doped compounds belonging to the 122 family of iron-based superconductors. The slope of the dispersion of this phonon mode is proportional to the square root of the shear modulus $C_{66}$ in the $q \rightarrow 0$ limit and, hence, sensitive to the tetragonal-to-orthorhombic structural phase transition occurring in these compounds. In contrast to a recent report for Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$ [F. Weber et al., Phys. Rev. B 98, 014516 (2018)], we find qualitative agreement between values of $C_{66}$ deduced from our experiments and those derived from measurements of the Youngs modulus in Ba$_{1-x}$(K,Na)$_x$Fe$_2$As$_2$ at optimal doping. These results provide an upper limit of about 50 Å for the nematic correlation length for the optimally hole-doped compounds. Furthermore, we also studied compounds at lower doping levels exhibiting the orthorhombic magnetic phase, where $C_{66}$ is not accessible by volume probes, as well as the C4 tetragonal magnetic phase.investigated

cond-mat.supr-con↗

arXiv2020

Synthesis and characterization of bulk Nd1-xSrxNiO2 and Nd1-xSrxNiO3

The recent reports of superconductivity in Nd1-xSrxNiO2/SrTiO3 heterostructures have reinvigorated interest in potential superconductivity of low-valence nickelates. Synthesis of Ni1+-containing compounds is notoriously difficult. In the current work, a combined sol-gel combustion and high-pressure annealing technique was employed to prepare polycrystalline perovskite Nd1-xSrxNiO3 (x = 0, 0.1 and 0.2). Metal nitrates and metal acetates were used as starting materials, and the latter were found to be superior to the former in terms of safety and reactivity. The Nd1-xSrxNiO3 compounds were subsequently reduced to Nd1 xSrxNiO2 using calcium hydride in a sealed, evacuated quartz tube. To understand the synthesis pathway, the evolution from NdNiO3 to NdNiO2 was monitored using in-situ synchrotron X ray diffraction during the reduction process. Electrical transport properties were consistent with an insulator-metal transition occurring between x = 0 and 0.1 for Nd1-xSrxNiO3. Superconductivity was not observed in our bulk samples of Nd1-xSrxNiO2. Neutron diffraction experiments at 3 K and 300 K were performed on Nd0.9Sr0.1NiO2, in which no magnetic Bragg reflections were observed, and the results of structural Rietveld refinement are provided.

cond-mat.mtrl-sci↗

arXiv2020

Intertwined density waves in a metallic nickelate

Nickelates are a rich class of materials, ranging from insulating magnets to superconductors. But for stoichiometric materials, insulating behavior is the norm, as for most late transition metal oxides. Notable exceptions are the 3D perovskite LaNiO$_3$, an unconventional paramagnetic metal, and the layered Ruddlesden-Popper phases R$_4$Ni$_3$O$_{10}$, (R=La, Pr, Nd). The latter are particularly intriguing because they exhibit an unusual metal-to-metal transition. Here, we demonstrate that this transition results from an incommensurate density wave with both charge and magnetic character that lies intermediate in behavior between the metallic density wave seen in chromium metal and the insulating stripes typically found in layered nickelates. As such, R$_4$Ni$_3$O$_{10}$, which appears to be the first known example of an itinerant spin density wave in a 3d transition metal oxide, represents an important bridge between the paramagnetism of 3D metallic LaNiO$_3$ at higher nickel valence and the polaronic behavior of quasi-2D R$_{2-x}$Sr$_x$NiO$_4$ at lower nickel valence.

cond-mat.str-el↗

arXiv2019

Spin stripe order in a square planar trilayer nickelate

Trilayer nickelates, which exhibit a high degree of orbital polarization combined with an electron count (d8.67) corresponding to overdoped cuprates, have been identified as a promising candidate platform for achieving high-Tc superconductivity. One such material, La4Ni3O8, undergoes a semiconductor-insulator transition at ~105 K, which was recently shown to arise from the formation of charge stripes. However, an outstanding issue has been the origin of an anomaly in the magnetic susceptibility at the transition and whether it signifies formation of spin stripes akin to single layer nickelates. Here we report single crystal neutron diffraction measurements (both polarized and unpolarized) that establish that the ground state is indeed magnetic. The ordering is modeled as antiferromagnetic spin stripes that are commensurate with the charge stripes, the magnetic ordering occurring in individual trilayers that are essentially uncorrelated along the crystallographic c-axis. Comparison of the charge and spin stripe order parameters reveals that, in contrast to single-layer nickelates such as La2-xSrxNiO4 as well as related quasi-2D oxides including manganites, cobaltates, and cuprates, these orders uniquely appear simultaneously, thus demonstrating a stronger coupling between spin and charge than in these related low-dimensional correlated oxides.

cond-mat.str-el↗

arXiv2019

Suppression of charge density wave order by disorder in Pd-intercalated ErTe$_3$

Disorder is generically anticipated to suppress long range charge density wave (CDW) order. We report transport, thermodynamic, and scattering experiments on Pd$_x$ErTe$_3$, a model CDW system with disorder induced by intercalation. The pristine parent compound ($x=0$) shows two separate, mutually perpendicular, incommensurate unidirectional CDW phases setting in at 270 K and 165 K. Here we track the suppression of signatures corresponding to these two parent transitions as the Pd concentration increases. At the largest values of $x$, we observe complete suppression of long range CDW order in favor of superconductivity. We also report evidence from electron and x-ray diffraction which suggests a tendency toward short-range ordering along both wavevectors which persists even well above the crossover temperature. Pd$_x$ErTe$_3$ provides a promising model system for the study of the interrelation of charge order and superconductivity in the presence of quenched disorder.

cond-mat.str-el↗

arXiv2018

Competing soft phonon modes at the charge-density-wave transitions in DyTe$_3$

The family of rare-earth tritellurides RTe$_3$ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [1]. What sets the heavier members of the RTe3 series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wavevector $q_{CDW,2}\parallel [100]$ of almost the same magnitude but orthogonal to the ordering wavevector $q_{CDW,1}\parallel [001]$ observed at higher temperatures [2]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of DyTe$_3$. In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at $T_{CDW,1}=308\,\rm{K}$. Surprisingly, these soft modes close to $q_{CDW,2}=(0.68,0,0)$ show strong softening near $T_{CDW,1}$ but do not exhibit any response to the lower-temperature transition at $T_{CDW,2}=68\,\rm{K}$. Our results indicate that the low-temperature CDW order is not just the 90° rotated analogue of the one appearing at high temperatures.

cond-mat.mtrl-sci↗

arXiv2018

Spectroscopic evidence for temperature-dependent convergence of light and heavy hole valence bands of PbQ (Q=Te, Se, S)

We have conducted temperature dependent Angle Resolved Photoemission Spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the \emph{light} hole upper valence bands (UVBs) and hitherto undetected \emph{heavy} hole lower valence bands (LVBs) in these materials. An unusual temperature dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is referred as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.

cond-mat.str-el↗

arXiv2016

Superconductivity and hybrid soft modes in TiSe$_2$

The competition between superconductivity and other ground states of solids is one of the challenging topics in condensed matter physics. Apart from high-temperature superconductors [1,2] this interplay also plays a central role in the layered transition-metal dichalcogenides, where superconductivity is stabilized by suppressing charge-density-wave order to zero temperature by intercalation [3] or applied pressure [4-7]. 1T-TiSe$_2$ forms a prime example, featuring superconducting domes on intercalation as well as under applied pressure. Here, we present high energy-resolution inelastic x-ray scattering measurements of the CDW soft phonon mode in intercalated Cu$_x$TiSe$_2$ and pressurized 1T-TiSe$_2$ along with detailed ab-initio calculations for the lattice dynamical properties and phonon-mediated superconductivity. We find that the intercalation-induced superconductivity can be explained by a solely phonon-mediated pairing mechanism, while this is not possible for the superconducting phase under pressure. We argue that a hybridization of phonon and exciton modes in the pairing mechanism is necessary to explain the full observed temperature-pressure-intercalation phase diagram. These results indicate that 1T-TiSe$_2$ under pressure is close to the elusive state of the excitonic insulator.

cond-mat.supr-con↗

arXiv2016

Superconducting energy gap of $2H$-NbSe$_2$ in phonon spectroscopy

We present a high energy-resolution inelastic x-ray scattering data investigation of the charge-density-wave (CDW) soft phonon mode upon entering the superconducting state in $2H$-NbSe$_2$. Measurements were done close to the CDW ordering wavevector $\mathbf{q}_{CDW}$ at $\mathbf{q}=\mathbf{q}_{CDW}+(0,0,l)$,$0.15\leq l \leq 0.5$, for $T=10\,\rm{K}$ (CDW order) and $3.8\,\rm{K}$ (CDW order + superconductivity). We observe changes of the phonon lineshape that are characteristic for systems with strong electron-phonon coupling in the presence of a superconducting energy gap $2Δ_c$ and from which we can demonstrate an $l$-dependence of the superconducting gap. Reversely, our data imply that the CDW energy gap is strongly localized along the $c^*$ direction. The confinement of the CDW gap to a very small momentum region explains the rather low competition and easy coexistence of CDW order and superconductivity in $2H$-NbSe$_2$. However, the energy gained by opening $Δ_{CDW}$ seems to be too small to be the driving force of the phase transition at $T_{CDW}=33\,\rm{K}$ , which is better described as an electron-phonon coupling driven structural phase transition.

cond-mat.supr-con↗