arXiv subjects

J. F. Mitchell

Publications and source records attributed to J. F. Mitchell.

At least 19 recordsLinked to original sources

arXiv2026

Layer Architecture Shapes Electronic, Magnetic, and Lattice Interactions in Ruddlesden-Popper Nickelates

The discovery of superconductivity in Ruddlesden-Popper nickelates has raised a central question: how does layer architecture shape the electronic, magnetic, and lattice interactions relevant to pairing? Here, we report a detailed comparative study of the two polymorphs of La3Ni2O7--the alternating monolayer-trilayer (LNO-1313) and bilayer (LNO-2222) structures--and the related trilayer compound La4Ni3O10, using both Ni L3- and O K-edge RIXS. We find that LNO-1313 and La4Ni3O10 share strikingly similar electronic, magnetic, and lattice excitations, whereas bilayer LNO-2222 exhibits distinct features. Compared to LNO-2222, LNO-1313 and La4Ni3O10 have weaker orbital polarization, enhanced 3d8L character, a reduced out-of-plane magnetic-exchange scale, and stronger EPC. Within an effective local-moment framework, an entangled-dimer scenario provides a natural description of the spin excitations generated by strong antiferromagnetic interlayer coupling. Its advantage over conventional spin-wave theory is clearest in bilayer LNO-2222, where the interlayer coupling dominates the intralayer interactions. These findings provide critical experimental constraints for future theoretical models for the low-energy physics relevant to superconductivity in these layered nickelates.

cond-mat.str-el↗

arXiv2026

Ultrafast Magneto-Pressure Spectroscopy and Control of Correlated Phases in a Trilayer Nickelate

Ultrafast spectroscopy under simultaneous high pressure and magnetic field provides a versatile approach for investigating pressure-driven electronic instabilities and correlated phases, and for probing potential bulk superconducting behavior under extreme conditions. However, such an experimental platform has yet to be implemented, standing as a roadblock to a fuller understanding of nonequilibrium superconductivity and vortex-controlled quasi-particle (QP) dynamics. Here, we bridge this capability gap by developing high pressure (up to 40 GPa), high magnetic field (up to 7 T), cryogenic (down to 5 K) femtosecond spectroscopy, and using it to probe magneto-pressure evolution of quasiparticle dynamics in the trilayer nickelate $\mathrm{Pr}_4\mathrm{Ni}_3\mathrm{O}_{10}$. We observe pronounced critical slowing down of QP relaxation at the charge-density-wave transition, which collapses under applied pressure. At higher pressures, the relaxation instead lengthens at low temperature, consistent with incipient superconducting correlations. However, the negligibel magnetic-field-dependence up to 7~T and absence of vortex-induced pre-bottleneck dynamics--robust signatures observed in our controlled bulk superconducting samples--indicates that any superconducting state under the present pressure conditions is likely non-bulk, filamentary, or strongly inhomogeneous. The magneto-pressure ultrafast capability opens a new avenue for resolving outstanding questions surrounding pressure-induced superconductivity and intertwined orders in correlated quantum materials.

cond-mat.supr-con↗

arXiv2026

Phonons reflect dynamic spin-state order in LaCoO$_3$

We investigate lattice dynamics in LaCoO$_3$ using inelastic neutron and x-ray scattering over $T = 2\mbox{-}650\,\mathrm{K}$, spanning the spin-state crossover at $T_{1} \approx 100\,\mathrm{K}$ and the insulator--metal transition at $T_{2} \approx 550\,\mathrm{K}$. Comparison with quasi-harmonic $ab-initio$ lattice-dynamical calculations helps reveal anomalous softening of a $\approx 10\,\mathrm{meV}$ oxygen phonon, confined to the temperature interval $T_{1} \leq T \leq T_{2}$ and localized in momentum space at $\boldsymbol{q}_{\mathrm{SSO}} = \left( \frac{1}{2},\frac{1}{2},\frac{1}{2} \right)_{c}$. This wave vector corresponds to the spin-state ordering originally proposed by Goodenough [J. Phys. Chem. Solids 6, 287-297 (1958)]. Our results therefore provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co$^{3+}$ states in LaCoO$_{3}$, linking spin-state fluctuations to anomalous phonon renormalization.

cond-mat.str-el↗

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↗

arXiv2026

Electronic layer decoupling driven by density-wave order in La$_4$Ni$_3$O$_{10}$

We probe the density-wave transition of the trilayer nickelate La$_4$Ni$_3$O$_{10}$ with polarization-resolved infrared spectroscopy. The low-energy electrodynamics is strongly anisotropic, with metallic in-plane and insulating out-of-plane character. In the ordered phase, the anisotropy grows more than an order of magnitude as the out-of-plane conductivity is sharply suppressed. We interpret this enhancement as an effective electronic decoupling of the Ni-O layers, driven by a spin-density-wave-induced redistribution of Ni-$d_{z^2}$ occupation within the trilayers. This electronic response is accompanied by clear shifts and splittings of the out-of-plane phonons, compatible with a density-wave instability of electronic origin.

cond-mat.str-el↗

arXiv2025

Domain wall induced topological Hall effect in the chiral-lattice ferromagnet Fe$_x$TaS$_2$

Magnetic topology and its associated emergent phenomena are central to realizing intriguing quantum states and spintronics functionalities. Designing spin textures to achieve strong and distinct electrical responses remains a significant challenge. Layered transition metal dichalcogenides offer a versatile platform for tailoring structural and magnetic properties, enabling access to a wide spectrum of topological magnetic states. Here, we report a domain-wall-driven, large, and tunable topological Hall effect (THE) in a non-centrosymmetric intercalated transition metal dichalcogenides series Fe$_x$TaS$_2$. By systematically varying the Fe intercalation level, we exert precise control over the magnetic ground states, allowing manipulation of the topological Hall effect. Real-space magnetic force microscopy (MFM) provides direct evidence of periodic magnetic stripe domain formation, confirming the microscopic origin of the observed topological transport phenomena. Our findings establish a promising way for tuning the topology of domains to generate substantial electromagnetic responses in layered magnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

Topological magneto-optics in the non-coplanar antiferromagnet Co_{1/3}NbS_2: Imaging and writing chiral magnetic domains

Despite its tiny net magnetization, the antiferromagnetic (AFM) van der Waals material Co$_{1/3}$NbS$_2$ exhibits a large transverse Hall conductivity $\sigma_{xy}$ even at zero applied magnetic field, which arises, as recently shown, from the topological nature of its non-coplanar ``tetrahedral'' AFM order. This triple-Q magnetic order can be regarded as the short-lengthscale limit of a magnetic skyrmion lattice, and has an intrinsic spin chirality. Here we show, using optical wavelengths spanning the ultraviolet to infrared (400-1000 nm), that magnetic circular dichroism (MCD) provides an incisive optical probe of the topological AFM order in Co$_{1/3}$NbS$_2$. Measurements as a continuous function of photon energy are directly compared with first-principles calculations, revealing the influence of the underlying quantum geometry on optical conductivity. Leveraging the power and flexibility of optical methods, we use scanning MCD microscopy to directly image chiral AFM domains, and demonstrate writing of chiral AFM domains.

cond-mat.mes-hall↗

arXiv2025

Lattice-charge coupling in a trilayer nickelate with intertwined density wave order

Intertwined charge and spin correlations are ubiquitous in a wide range of transition metal oxides and are often perceived as intimately related to unconventional superconductivity. Theoretically envisioned as driven by strong electronic correlations, the intertwined order is usually found to be strongly coupled to the lattice as signaled by pronounced phonon softening. Recently, both charge/spin density waves (CDW/SDW) and superconductivity have been discovered in several Ruddlesden-Popper (RP) nickelates, in particular trilayer nickelates RE4Ni3O10 (RE=Pr, La). The nature of the intertwined order and the role of lattice-charge coupling are at the heart of the debate about these materials. Using inelastic X-ray scattering, we mapped the phonon dispersions in RE4Ni3O10 and found no evidence of phonon softening near the CDW wavevector over a wide temperature range. Calculations of the electronic susceptibility revealed a peak at the observed SDW ordering vector but not at the CDW wavevector. The absence of phonon softening is in sharp contrast to that in canonical oxide materials, notably cuprates. Our experimental and theoretical findings highlight the crucial role of the spin degree of freedom and establish a foundation for understanding the interplay between superconductivity and density-wave transitions in RP nickelate superconductors and beyond.

cond-mat.str-el↗

arXiv2025

Pressure tuning of Kitaev spin liquid candidate Na$_3$Co$_2$SbO$_6$

The search for Kitaev's quantum spin liquid (KQSL) state in real materials has recently expanded with the prediction that honeycomb lattices of divalent, high-spin cobalt ions could host the dominant bond-dependent exchange interactions required to stabilize the elusive entangled quantum state. The layered honeycomb Na$_3$Co$_2$SbO$_6$ has been singled out as a leading candidate provided that the trigonal crystal field acting on Co $3d$ orbitals, which enhances non-Kitaev exchange interactions between $J_{\rm eff}=\frac{1}{2}$ spin-orbital pseudospins, is reduced. We find that applied pressure leads to anisotropic compression of the layered structure, significantly reducing the trigonal distortion of CoO$_6$ octahedra. A strong enhancement of ferromagnetic correlations between pseudospins is observed in the spin-polarized (3 Tesla) phase up to about 60 GPa. Higher pressures drive a spin transition into a low-spin state destroying the $J_{\rm eff}=\frac{1}{2}$ local moments required to map the spin Hamiltonian into Kitaev's model. The spin transition strongly suppresses the low-temperature magnetic susceptibility and appears to stabilize a paramagnetic phase driven by frustration. Although applied pressure fails to realize a KQSL state, the possible emergence of frustrated magnetism of localized, low-spin $S=\frac{1}{2}$ moments opens the door for exploration of novel magnetic quantum states in compressed honeycomb lattices of divalent cobaltates.

cond-mat.str-el↗

arXiv2025

Universal electronic structure of multi-layered nickelates via oxygen-centered planar orbitals

Superconductivity has been demonstrated in the family of multi-layered nickelates La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$. Key questions remain open regarding the low-energy electronic states that support superconductivity in these compounds. Here we take advantage of the natural polymorphism between bilayer (2222) and alternating monolayer-trilayer (1313) stacking sequences that arises in bulk La$_3$Ni$_2$O$_7$ crystals, and by employing angle-resolved photoemission spectroscopy (ARPES) we identify a universal low-energy electronic structure in this family of materials. We observe the fingerprint of a doping-dependent spin-density wave (SDW) instability -- strong and coherent enough to reconstruct the Fermi surface, both by gapping out regions of the low-energy electronic structure as well as translating the $\beta$ pocket by a vector $Q_{t\beta}$ consistent with the results of previous neutron and x-ray scattering experiments. Using an effective tight-binding model, we simulate the spectral weight distribution observed in our ARPES dichroism experiments and establish that the low-energy electronic phenomenology is dominated by oxygen-centered planar orbitals, which evolve from the $d_{3x^2-r^2}$ and $d_{3y^2-r^2}$ symmetry characteristic of 3-spin polarons (3SP) to the familiar $d_{x^2-y^2}$ Zhang-Rice singlets (ZRS) that support high-temperature superconductivity in cuprates. By inclusion of magnetic moments on plaquettes of oxygen orbitals in our model, we show that ZRS-like states mediate the SDW. Combined with the observation that oxygen annealing is required to induce superconductivity in both thin films and bulk La$_3$Ni$_2$O$_7$, this demonstrates that the ZRS population dictates whether the ground state favors density-wave order or superconductivity -- with hole doping suppressing the former and stabilizing the latter, as in the cuprates.

cond-mat.supr-con↗

arXiv2024

Tunable topological transitions in the frustrated magnet HoAgGe

The kagome lattice, known for its strong frustration in two dimensions, hosts a variety of exotic magnetic and electronic states. A variation of this geometry, where the triangular motifs are twisted to further reduce symmetry, has recently revealed even more complex physics. HoAgGe exemplifies such a structure, with magnetic and electronic properties believed to be driven by strong in-plane anisotropy of the Ho spins, effectively acting as a two-dimensional spin ice. In this study, using a combination of magnetization, Hall conductivity measurements, and density functional theory calculations, we demonstrate how various spin-ice states, stabilized by external magnetic fields, influence the Fermi surface topology. More interestingly, we observe sharp transitions in Hall conductivity without concurrent changes in magnetization when an external magnetic field is applied along a particular crystallographic direction, underscoring the role of strong magnetic frustration and providing a new platform for exploring the interplay between magnetic frustration, electronic topology, and crystalline symmetry. These results also highlight the limitations of a simple spin-ice model, suggesting that a more sophisticated framework is necessary to capture the subtle experimental nuances observed.

cond-mat.str-el↗

arXiv2024

Non-bulk Superconductivity in Pr$_4$Ni$_3$O$_{10}$ Single Crystals Under Pressure

Magneto-transport measurements of Pr$_4$Ni$_3$O$_{10}$ single crystals performed under externally applied pressures up to 73 GPa in diamond anvil cells with either KBr or Nujol oil as pressure media yield signatures of superconductivity with a maximum onset temperature of approximately 31 K. True zero resistance was not observed, consistent with a non-percolating superconducting volume fraction. Magnetization measurements provided corroborating evidence of superconductivity, with a pressure-dependent diamagnetic signal occurring below the onset temperature, and an estimate from the absolute value of the susceptibility suggests a superconducting volume fraction on the order of 10%. We observe sample-to-sample variations in the magnitude and pressure dependence of Tc as well as a dependence on the configuration of electrical contacts on a given sample. Possible causes of this behavior may be significant inhomogeneities in the pressure and/or damage to the samples induced by the pressure media as well as inhomogeneities in the crystals themselves. The results imply that our as-grown Pr$_4$Ni$_3$O$_{10}$ single crystals are not bulk superconductors but that there is a minority structure present within the crystals that is indeed superconducting.

cond-mat.supr-con↗

arXiv2024

Altermagnetism in the layered intercalated transition metal dichalcogenide CoNb$_4$Se$_8$

Altermagnets (AMs) are a new class of magnetic materials that combine the beneficial spintronics properties of ferromagnets and antiferromagnets, garnering significant attention recently. Here, we have identified altermagnetism in a layered intercalated transition metal diselenide, CoNb$_4$Se$_8$, which crystallizes with an ordered sublattice of intercalated Co atoms between NbSe$_2$ layers. Single crystals are synthesized, and the structural characterizations are performed using single crystal diffraction and scanning tunneling microscopy. Magnetic measurements reveal easy-axis antiferromagnetism below 168 K. Density functional theory (DFT) calculations indicate that A-type antiferromagnetic ordering with easy-axis spin direction is the ground state, which is verified through single crystal neutron diffraction experiments. Electronic band structure calculations in this magnetic state display spin-split bands, confirming altermagnetism in this compound. The layered structure of CoNb$_4$Se$_8$ presents a promising platform for testing various predicted properties associated with altermagnetism.

cond-mat.str-el↗

arXiv2024

Fermi Surface Topology and Magneto-transport Properties of Superconducting Pd$_3$Bi$_2$Se$_2$

Pd$_3$Bi$_2$Se$_2$ is a rare realization of a superconducting metal with a non-zero $Z_2$ topological invariant. We report the growth of high-quality single crystals of layered Pd$_3$Bi$_2$Se$_2$ with a superconducting transition at $T_c$ ~ 0.80 K and upper critical fields of ~10 mT and ~5 mT for the in-plane and out-of-plane directions, respectively. Our density functional theory (DFT) calculations reveal three pairs of doubly degenerate bands crossing the Fermi level, all displaying clear three-dimensional dispersion consistent with the overall low electronic anisotropy (<2). The multiband electronic nature of Pd$_3$Bi$_2$Se$_2$ is evident in magneto-transport measurements, yielding a sign-changing Hall resistivity at low temperatures. The magnetoresistance is non-saturating and follows Kohler's scaling rule. We interpret the magneto-transport data in terms of open orbits that are revealed in the DFT-calculated Fermi surface. de Haas-van Alphen (dHvA) oscillation measurements using torque magnetometry on single crystals yield four frequencies for out-of-plane fields: $F_\alpha = (150 \pm 26)$T, $F_\beta = (293 \pm 10)$T, $F_\gamma = (375 \pm 20)$T, and $F_\eta = (1017 \pm 12)$T, with the low frequency dominating the spectrum. Through the measurement of angular dependent dHvA oscillations and DFT calculations, we identify the $F_\alpha$ frequency with an approximately ellipsoidal electron pocket centered on the $L_2$ point of the Brillouin zone. Lifshitz-Kosevich analysis of the dHvA oscillations reveals a small cyclotron effective mass: $m^* = (0.11 \pm 0.02) m_0$ and a nontrivial Berry phase for the dominant orbit. The presence of nontrivial topology in a bulk superconductor positions Pd$_3$Bi$_2$Se$_2$ as a potential candidate for exploring topological superconductivity.

cond-mat.str-el↗

arXiv2024

Origin of anomalous magnetotransport in kagome superconductors AV$_{3}$Sb$_{5}$ (A=K,Rb,Cs)

Multiple anomalous features in electronic spectra of metals with kagome lattice structure -- van Hove singularities, Dirac points, and flat bands -- imply that materials containing this structural motif may lie at a nexus of topological and correlated electron physics. Due to the prospects of such exceptional electronic behavior, the recent discovery of superconductivity coexisting with charge-density wave (CDW) order in the layered kagome metals AV$_{3}$Sb$_{5}$ (A=K,Rb,Cs) has attracted considerable attention. Notably, these kagome metals express unconventional magnetotransport behavior, including a linear-in-H diagonal resistivity at low fields, and an even more peculiar, nonmonotonic sign-changing behavior of the Hall resistivity, which has been speculated to arise from a chiral CDW. We argue here that this unusual magnetotransport derives not from such unconventional phenomena, but rather from the unique fermiology of the AV$_{3}$Sb$_{5}$ materials. Specifically, it is caused by a large, concave hexagonal Fermi surface sheet formed in the close proximity to the van Hove singularities, which is backfolded into a small hexagonal sheet and two large triangular sheets in the CDW state. We introduce a model of the electronic structure of these Fermi surface sheets that allows for a full analytical treatment within Boltzmann kinetic theory and that enables semi-quantitative fits of our transport data. Specifically, we find that the anomalous magnetotransport behavior is caused by the confluence of strong reduction of the Fermi velocity near the van Hove singularities located near the vertices of the hexagonal sheet and sharp corners in Fermi surface generated by the CDW reconstruction.

cond-mat.supr-con↗

arXiv2024

Low temperature dynamic polaron liquid in a manganite exhibiting colossal magnetoresistance

Polarons - fermionic charge carriers bearing a strong companion lattice deformation - exhibit a natural tendency for self-localization due to the recursive interaction between electrons and the lattice. While polarons are ubiquitous in insulators, how they evolve in transitions to metallic and superconducting states in quantum materials remains an open question. Here, we use resonant inelastic x-ray scattering (RIXS) to track the electron-lattice coupling in the colossal magneto-resistive bi-layer manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ across its metal-to-insulator transition. The response in the insulating high-temperature state features harmonic emissions of a dispersionless oxygen phonon at small energy transfer. Upon cooling into the metallic state, we observe a drastic redistribution of spectral weight from the region of these harmonic emissions to a broad high energy continuum. In concert with theoretical calculations, we show that this evolution implies a shift in electron-lattice coupling from static to dynamic lattice distortions that leads to a distinct polaronic ground state in the low temperature metallic phase - a dynamic polaron liquid.

cond-mat.str-el↗

arXiv2023

Pressure-dependent "Insulator-Metal-Insulator" Behavior in Sr-doped La$_3$Ni$_2$O$_7$

Recently, superconductivity at high temperatures has been observed in bulk La$_3$Ni$_2$O$_{7-δ}$ under high pressure. However, the attainment of high-purity La$_3$Ni$_2$O$_{7-δ}$ single crystals, exhibiting controlled and homogeneous stoichiometry through the post-annealing process in an oxygen-rich floating zone furnace, remains a formidable challenge. Here, we report the crystal structure and physical properties of single crystals of Sr-doped La$_3$Ni$_2$O$_7$ synthesized at high pressure (20 GPa) and high temperature (1400 °C). Through single crystal X-ray diffraction, we showed that high-pressure-synthesized paramagnetic Sr-doped La$_3$Ni$_2$O$_7$ crystallizes in an orthorhombic structure with Ni-O-Ni bond angles of 173.4(2)°out-of-plane and 175.0(2)°and 176.7(2)°in plane. The substitution of Sr alters in band filling and the ratio of Ni$^{2+}$/Ni$^{3+}$ in Sr-doped La$_3$Ni$_2$O$_7$, aligning them with those of "La$_3$Ni$_2$O$_{7.05}$", thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr-doped La$_3$Ni$_2$O$_7$ exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr-doped La$_3$Ni$_2$O$_7$ transits back from a metal-like behavior to an insulator. The insulator-metal-insulator trend under high pressure dramatically differs from the behavior of the parent compound La$_3$Ni$_2$O$_{7-δ}$, despite their similar behavior in the low-pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates.

cond-mat.supr-con↗

arXiv2023

Polymorphism in Ruddlesden-Popper $La_{3}Ni_{2}O_{7}$: Discovery of a Hidden Phase with Distinctive Layer Stacking

We report the discovery of a novel form of Ruddlesden-Popper (RP) oxide, which stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase $La_{3}Ni_{2}O_{7}$adopts a novel stacking sequence in which single and trilayer blocks of $NiO_{6}$ octahedra alternate in a 1313 sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imcm variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with onset at T = 134 K, which lies intermediate between that of the standard 2222 polymorph of $La_{3}Ni_{2}O_{7}$ (space group Amam) and the trilayer RP phase, $La_{4}Ni_{3}O_{10}$. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity with $T_{c}$ as high as 80 K above 14 GPa in bilayer $La_{3}Ni_{2}O_{7}$.

cond-mat.supr-con↗