arXiv subjects

S. Chi

Publications and source records attributed to S. Chi.

At least 19 recordsLinked to original sources

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

First Order Preemptive Ising-nematic Transition in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$

Employing inelastic X-ray scattering and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$. Upon cooling, the nematic order undergoes a strongly first-order phase transition followed by a second-order magnetic transition at $T_{\textrm{N}}$ $\approx$ 34.6 K. The temperature difference between these two phase transitions is $\sim$ 1 K. The observed phenomenon can be attributed to a distinctive first-order preemptive Ising-nematic transition, a characteristic unique to a quasi-two-dimensional scenario marked by strong out-of-plane spatial anisotropy due to weak coupling. Our studies establish K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$ as the first material in the family of iron pnictides and chalcogenides that possesses a nematic tricritical point preceding the magnetic one upon decreasing nematic coupling.

cond-mat.str-el↗

arXiv2023

Quantum to classical crossover in generalized spin systems -- the temperature-dependent spin dynamics of FeI$_2$

Simulating quantum spin systems at finite temperatures is an open challenge in many-body physics. This work studies the temperature-dependent spin dynamics of a pivotal compound, FeI$_2$, to determine if universal quantum effects can be accounted for by a phenomenological renormalization of the dynamical spin structure factor $S(\mathbf{q}, \omega)$ measured by inelastic neutron scattering. Renormalization schemes based on the quantum-to-classical correspondence principle are commonly applied at low temperatures to the harmonic oscillators describing normal modes. However, it is not clear how to extend this renormalization to arbitrarily high temperatures. Here we introduce a temperature-dependent normalization of the classical moments, whose magnitude is determined by imposing the quantum sum rule, i.e. $\int d\omega d\mathbf{q} S(\mathbf{q}, \omega) = N_S S (S+1)$ for $N_S$ dipolar magnetic moments. We show that this simple renormalization scheme significantly improves the agreement between the calculated and measured $S(\mathbf{q}, \omega)$ for FeI$_{2}$ at all temperatures. Due to the coupled dynamics of dipolar and quadrupolar moments in that material, this renormalization procedure is extended to classical theories based on SU(3) coherent states, and by extension, to any SU(N) coherent state representation of local multipolar moments.

cond-mat.str-el↗

arXiv2023

Helical magnetic state in the vicinity of the pressure-induced superconducting phase in MnP

MnP is a metal that shows successive magnetic transitions from paramagnetic to ferromagnetic and helical magnetic phases at ambient pressure with decreasing temperature. With applied pressure, the magnetic transition temperatures decrease and superconductivity appears around 8 GPa where the magnetic order is fully suppressed and the quantum critical behavior is observed. These results suggest that MnP is an unconventional superconductor in which magnetic fluctuations may be relevant to the superconducting pairing mechanism. In order to elucidate the magnetic ground state adjacent to the superconducting phase first discovered in Mn-based materials, high-pressure neutron diffraction measurements have been performed in hydrostatic pressure up to 7.5 GPa. The helical magnetic structure with the propagation vector along the $b$ axis, reported previously at 3.8 GPa, was found to be robust up to 7.5 GPa. First principles and classical Monte Carlo calculations have also been performed to understand how the pressure-driven magnetic phase transitions are coupled with change of the exchange interactions. The calculations, which qualitatively reproduce the magnetic structures as a function of pressure, suggest that the exchange interactions change drastically with applied pressure and the further-neighbor interactions become more influential at high pressures. Combining the experimental and theoretical results, we describe the detail of exchange interactions in the vicinity of the superconducting phase which is critical to understand the pairing mechanism of the unconventional superconductivity in MnP.

cond-mat.mtrl-sci↗

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↗

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

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

Searching for obscured AGN in z $\sim$ 2 submillimetre galaxies

Submillimetre-selected galaxies (SMGs) at high redshift ($z$ $\sim$ 2) are potential host galaxies of active galactic nuclei (AGN). If the local Universe is a good guide, $\sim$ 50$\%$ of the obscured AGN amongst the SMG population could be missed even in the deepest X-ray surveys. Radio observations are insensitive to obscuration; therefore, very long baseline interferometry (VLBI) can be used as a tool to identify AGN in obscured systems. A well-established upper limit to the brightness temperature of 10$^5$ K exists in star-forming systems, thus VLBI observations can distinguish AGN from star-forming systems via brightness temperature measurements. We present 1.6 GHz European VLBI Network (EVN) observations of four SMGs (with measured redshifts) to search for evidence of compact radio components associated with AGN cores. For two of the sources, e-MERLIN images are also presented. Out of the four SMGs observed, we detect one source, J123555.14, that has an integrated EVN flux density of 201 $\pm$ 15.2 $\mu$Jy, corresponding to a brightness temperature of 5.2 $\pm$ 0.7 $\times$ 10$^5$ K. We therefore identify that the radio emission from J123555.14 is associated with an AGN. We do not detect compact radio emission from a possible AGN in the remaining sources (J123600.10, J131225.73, and J163650.43). In the case of J131225.73, this is particularly surprising, and the data suggest that this may be an extended, jet-dominated AGN that is resolved by VLBI. Since the morphology of the faint radio source population is still largely unknown at these scales, it is possible that with a $\sim$ 10 mas resolution, VLBI misses (or resolves) many radio AGN extended on kiloparsec scales.

astro-ph.GA↗

arXiv2018

Spin-Liquid-Like State in Pure and Mn-Doped TbInO3 with Nearly Triangular Lattice

Inelastic neutron scattering studies in single crystals of TbInO3 and TbIn0.95Mn0.05O3 with nearly-triangular antiferromagnetic lattice are reported. At low energies, a broad and apparently gapless continuum of magnetic excitations, located at the triangular lattice (TL) Brillouin zone boundary, is observed. The data are well described by the uncorrelated nearest-neighbor valence bonds model. At higher energies, a broad excitation branch dispersing from the TL zone boundary is observed. No signs of static magnetic order are found down to the temperatures two orders of magnitude smaller than the effective interaction energy. The fluctuating magnetic moment exceeds two thirds of the Tb3+ free-ion value and is confined to the TL plane. These observations are consistent with a TL-based spin liquid state in TbInO3.

cond-mat.str-el↗

arXiv2018

Lattice distortion in the spin-orbital entangled state in RVO3 perovskites

We report a thorough study of Y$_{0.7}$La$_{0.3}$VO$_3$ single crystals by measuring magnetic properties, specific heat, thermal conductivity, x-ray and neutron diffraction with the motivation of revealing the lattice response to the spin-orbital entanglement in \textit{R}VO$_3$. Upon cooling from room temperature, the orbitally disordered paramagnetic state changes around T*$\sim$220\,K to spin-orbital entangled state which is then followed by a transition at T$_N$=116\,K to C-type orbital ordered (OO) and G-type antiferromagnetic ordered (AF) ground state. In the temperature interval T$_N<T<T^*$, the VO$_{6/2}$ octahedra have two comparable in-plane V-O bonds which are longer than the out-of-plane V-O1 bond. This local structural distortion supports the spin-orbital entanglement of partially filled and degenerate yz/zx orbitals. However, this distortion is incompatible with the steric octahedral site distortion intrinsic to orthorhombic perovskites. Their competition induces a second order transition from the spin-orbital entangled state to C-OO/G-AF ground state where the long range OO suppresses the spin-orbital entanglement. Our analysis suggests that the spin-orbital entangled state and G-OO are comparable in energy and compete with each other. Rare earth site disorder favors the spin-orbital entanglement rather than a cooperative Jahn-Teller distortion. The results also indicate for LaVO$_3$ a C-OO/G-AF state in T$_t$\,$\leq$\,T\,$\leq$T$_N$ and an orbital flipping transition at T$_t$.

cond-mat.str-el↗

arXiv2017

Influence of Spin Orbit Coupling in the Iron-Based Superconductors

We report on the influence of spin-orbit coupling (SOC) in the Fe-based superconductors (FeSCs) via application of circularly-polarized spin and angle-resolved photoemission spectroscopy. We combine this technique in representative members of both the Fe-pnictides and Fe-chalcogenides with ab initio density functional theory and tight-binding calculations to establish an ubiquitous modification of the electronic structure in these materials imbued by SOC. The influence of SOC is found to be concentrated on the hole pockets where the superconducting gap is generally found to be largest. This result contests descriptions of superconductivity in these materials in terms of pure spin-singlet eigenstates, raising questions regarding the possible pairing mechanisms and role of SOC therein.

cond-mat.supr-con↗

arXiv2017

Competing spin density wave, collinear, and helical magnetism in Fe1+xTe

The Fe1+xTe phase diagram consists of two distinct magnetic structures with collinear order present at low interstitial iron concentrations and a helical phase at large values of x with these phases separated by a Lifshitz point. We use unpolarized single crystal diffraction to confirm the helical phase for large interstitial iron concentrations and polarized single crystal diffraction to demonstrate the collinear order for the iron deficient side of the Fe1+xTe phase diagram. Polarized neutron inelastic scattering show that the fluctuations associated with this collinear order are predominately transverse at low energy transfers, consistent with a localized magnetic moment picture. We then apply neutron inelastic scattering and polarization analysis to investigate the dynamics and structure near the boundary between collinear and helical order in the Fe1+xTe phase diagram. We first show that the phase separating collinear and helical order is characterized by a spin-density wave with a single propagation wave vector of (~ 0.45, 0, 0.5). We do not observe harmonics or the presence of a charge density wave. The magnetic fluctuations associated with this wavevector are different from the collinear phase being strongly longitudinal in nature and correlated anisotropically in the (H,K) plane. The excitations preserve the C4 symmetry of the lattice, but display different widths in momentum along the two tetragonal directions at low energy transfers. While the low energy excitations and minimal magnetic phase diagram can be understood in terms of localized interactions, we suggest that the presence of density wave phase implies the importance of electronic and orbital properties.

cond-mat.str-el↗

arXiv2016

Thermal Conductivity of the Iron-Based Superconductor FeSe : Nodeless Gap with Strong Two-Band Character

The thermal conductivity of the iron-based superconductor FeSe was measured at temperatures down to 50 mK in magnetic fields up to 17 T. In zero magnetic field, the electronic residual linear term in the T = 0 limit, \kappa_0/T, is vanishingly small. Application of a magnetic field H causes no increase in \kappa_0/T initially. Those two observations show that there are no zero-energy quasiparticles that carry heat and therefore no nodes in the superconducting gap of FeSe. The full field dependence of \kappa_0/T has the classic shape of a two-band superconductor, such as MgB2: it rises exponentially at very low field, with a characteristic field H* << Hc2, and then more slowly up to the upper critical field Hc2. This shows that the superconducting gap is very small on one of the pockets in the Fermi surface of FeSe.

cond-mat.supr-con↗

arXiv2016

Pressure dependence of the magnetic ground states in MnP

MnP, a superconductor under pressure, exhibits a ferromagnetic order below TC~290 K followed by a helical order with the spins lying in the ab plane and the helical rotation propagating along the c axis below Ts~50 K at ambient pressure. We performed single crystal neutron diffraction experiments to determine the magnetic ground states under pressure. Both TC and Ts are gradually suppressed with increasing pressure and the helical order disappears at ~1.2 GPa. At intermediate pressures of 1.8 and 2.0 GPa, the ferromagnetic order first develops and changes to a conical or two-phase (ferromagnetic and helical) structure with the propagation along the b axis below a characteristic temperature. At 3.8 GPa, a helical magnetic order appears below 208 K, which hosts the spins in the ac plane and the propagation along the b axis. The period of this b axis modulation is shorter than that at 1.8 GPa. Our results indicate that the magnetic phase in the vicinity of the superconducting phase may have a helical magnetic correlation along the b axis.

cond-mat.supr-con↗

arXiv2014

The effects of Co3O4 on the Structure and Unusual Magnetism of LaCoO3

Bulk La_wCoO3 particles with w=1.1, 1.0, 0.9, 0.8, and 0.7 were synthesized using starting materials with varying molar ratios of La2O3 and Co3O4. The resulting particles are characterized as LaCoO3 crystals interfaced with a crystalline Co3O4 phase. X-ray and neutron scattering data show little effect on the average structure and lattice parameters of the LaCoO3 phase resulting from the Co3O4 content, but magnetization data indicate that the amount of Co3O4 strongly affects the ferromagnetic ordering at the interfaces below T_C ~89K. In addition to ferromagnetic long-range order, LaCoO3 exhibits antiferromagnetic behavior with an unusual temperature dependence. The magnetization for fields 20 Oe < H < 5 kOe is fit to a combination of a power law ((T-T_C)/T_C)^beta behavior representing the ferromagnetic long-range order and sigmoid-convoluted Curie-Weiss-like behavior representing the antiferromagnetic behavior. The critical exponent beta=0.63 +- 0.02 is consistent with 2D (surface) ordering. Increased Co3O4 correlates well to increased ferromagnetism. The weakening of the antiferromagnetism below T ~ 40K is a consequence of the lattice reaching a critical rhombahedral distortion as T is decreased for core regions far from the Co3O4 interfaces. We introduce a model that describes the ferromagnetic behavior of the interface regions and the unusual antiferromagnetism of the core regions.

cond-mat.str-el↗

arXiv2014

The unusual magnetism of nanoparticle LaCoO3

Bulk and nanoparticle powders of LaCoO3 (LCO) were synthesized, and their magnetic and structural properties were studied using SQUID magnetometry and neutron diffraction. The bulk and large nanoparticles exhibit weak ferromagnetism (FM) below T ~ 85K and a crossover from strong to weak antiferromagnetic (AFM) correlations near a transition expressed in the lattice parameters, To ~ 40K. This crossover does not occur in the smallest nanoparticles; instead, the magnetic behavior is predominantly ferromagnetic. The amount of FM in the nanoparticles depends on the amount of Co3O4 impurity phase, which induces tensile strain on the LCO lattice. A core-interface model is introduced, with the core region exhibiting the AFM crossover and with FM in the interface region near surfaces and impurity phases.

cond-mat.str-el↗