arXiv subjects

Stephen D. Wilson

Publications and source records attributed to Stephen D. Wilson.

At least 181 records · Page 10Linked to original sources

arXiv2019

Evolution of structure and magnetism across the metal-insulator transition in the pyrochlore iridate $($Nd$_{1-x}$Ca$_x)_2$Ir$_2$O$_7$

We report on the evolution of the thermal metal-insulator transition in polycrystalline samples of Nd$_2$Ir$_2$O$_7$ upon hole-doping via substitution of Ca$^{2+}$ for Nd$^{3+}$. Ca substitution mediates a filling-controlled Mott-like transition with minimal resolvable structural changes and without altering site symmetry. Local structure confirms that Ca substitution does not result in local chemical phase separation, and absorption spectroscopy establishes that Ir cations maintain a spin-orbit entangled electronic configuration. The metal-insulator transition coincides with antiferromagnetic ordering on the Ir sublattice for all measured samples, and both decrease in onset temperature with Ca content. Weak low-temperature upturns in susceptibility and resistivity for samples with high Ca content suggest that Nd sublattice antiferromagnetism continues to couple to carriers in the metallic regime.

cond-mat.mtrl-sci↗

arXiv2019

Doping induced Mott collapse and the density wave instabilities in (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$

The path from a Mott insulating phase to high temperature superconductivity encounters a rich set of unconventional phenomena involving the insulator-to-metal transition (IMT) such as emergent electronic orders and pseudogaps that ultimately affect the condensation of Cooper pairs. A huge hindrance to understanding the origin of these phenomena in the curates is the difficulty in accessing doping levels near the parent state. Recently, the J$_{eff}$=1/2 Mott state of the perovskite strontium iridates has revealed intriguing parallels to the cuprates, with the advantage that it provides unique access to the Mott transition. Here, we exploit this accessibility to study the IMT and the possible nearby electronic orders in the electron-doped bilayer iridate (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$. Using spectroscopic imaging scanning tunneling microscopy, we image the La dopants in the top as well as the interlayer SrO planes. Surprisingly, we find a disproportionate distribution of La in these layers with the interlayer La being primarily responsible for the IMT, thereby revealing the distinct site-dependent effects of dopants on the electronic properties of bilayer systems. Furthermore, we discover the coexistence of two electronic orders generated by electron doping: a unidirectional electronic order with a concomitant structural distortion; and local resonant states forming a checkerboard-like pattern trapped by La. This provides evidence that multiple charge orders may exist simultaneously in Mott systems, even with only one band crossing the Fermi energy.

cond-mat.str-el↗

arXiv2019

Overdamped antiferromagnetic strange metal state in Sr$_3$IrRuO$_7$

The unconventional electronic ground state of Sr$_3$IrRuO$_7$ is explored via resonant x-ray scattering techniques and angle-resolved photoemission measurements. As the Ru content approaches $x=0.5$ in Sr$_3$(Ir$_{1-x}$Ru$_x$)$_2$O$_7$, intermediate to the $J_{eff}=1/2$ Mott state in Sr$_3$Ir$_2$O$_7$ and the quantum critical metal in Sr$_3$Ru$_2$O$_7$, a thermodynamically distinct metallic state emerges. The electronic structure of this intermediate phase lacks coherent quasiparticles, and charge transport exhibits a linear temperature dependence over a wide range of temperatures. Spin dynamics associated with the long-range antiferromagnetism of this phase show nearly local, overdamped magnetic excitations and an anomalously large energy scale of 200 meV---an energy far in excess of exchange energies present within either the Sr$_3$Ir$_2$O$_7$ or Sr$_3$Ru$_2$O$_7$ solid-solution endpoints. Overdamped quasiparticle dynamics driven by strong spin-charge coupling are proposed to explain the incoherent spectral features of the strange metal state in Sr$_3$IrRuO$_7$.

cond-mat.str-el↗

arXiv2019

High-Pressure Laser Floating Zone Furnace

The floating zone technique is a well-established single crystal growth method in materials research, able to produce volumetrically large specimens with extremely high purities. However, traditional furnace designs have relied on heating from high-powered bulb sources in combination with parabolic mirrors, and hence are constrained to transparent growth chambers with large solid angles of optical access. This results in a stark limitation on achievable processing gas pressures, and in turn renders a range of compounds unsuitable for crystal growth by the floating zone technique, either due to excessive volatility or due to metastability. Here, we demonstrate a novel high-pressure laser-based floating zone system (HP-LFZ). The use of lasers for heating allows implementation of a high-strength metal growth chamber, permitting greatly enhanced processing pressures over conventional mirror-based designs, with the current design allowing for pressures up to 1000 bar. We demonstrate a series of example single crystal growths using this design in pressures up to 675 bar, a significant increase over processing pressures attainable in commercially available floating zone systems. The general utility of the HP-LFZ is also illustrated via growths of a range of complex oxides.

cond-mat.mtrl-sci↗

arXiv2019

Polarized Raman spectroscopy study of metallic $(Sr_{1-x}La_{x})_{3}Ir_{2}O_{7}$: a consistent picture of disorder-interrupted unidirectional charge order

We have used rotational anisotropic polarized Raman spectroscopy to study the symmetries, the temperature and the doping dependence of the charge ordered state in metallic $(Sr_{1-x}La_{x})_{3}Ir_{2}O_{7}$. Although the Raman probe size is greater than the charge ordering length, we establish that the charge ordering breaks the fourfold rotational symmetry of the underlying tetragonal crystal lattice into twofold, as well as the translational symmetry, and forms short-range domains with $90^{\circ}$ rotated charge order wave vectors, as soon as the charge order sets in below $T_{CO} = \sim$ 200K and across the doping-induced insulator metal transition. We observe that this charge order mode frequency remains nearly constant over a wide temperature range and up to the highest doping level. These above features are highly reminiscent of the ubiquitous unidirectional charge order in underdoped high-$T_C$ copper-oxide-based superconductors (cuprates). We further resolve that the charge order damping rate diverges when approaching $T_{CO}$ from below and increases significantly as increasing the La doping level, which resembles the behaviors for a disorder-interrupted ordered phase and has not been observed for the charge order in cuprates.

cond-mat.str-el↗

arXiv2019

Deciphering structural and magnetic disorder in the chiral skyrmion host materials Co$_x$Zn$_y$Mn$_z$ ($x+y+z=20$)

Co$_x$Zn$_y$Mn$_z$ ($x+y+z=20$) compounds crystallizing in the chiral $β$-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, CoxZnyMnz compounds have also been found to host metastable non-equilibrium skyrmion lattices in a broad temperature and field range, including down to zero-field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and DC and AC magnetic measurements, to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site-preference for the diamondoid 8c site in the crystal structure, while Mn tends to share the geometrically frustrated 12d site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich 8c sublattice and the Mn on the 12d sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 100K to 470K) with a moment around 1 $μ_B$/atom and maintains this order to low temperature. The Mn-rich sublattice holds larger moments (about 3 $μ_B$ which remain fluctuating below the Co moment ordering temperature. At lower temperature, the fluctuating Mn moments freeze into a reentrant disordered cluster-glass state with no net moment, while the Co moments maintain order. This two-sublattice behavior allows for the observed coexistence of strong magnetic disorder and ordered magnetic states such as helimagnetism and skyrmion lattices.

cond-mat.mtrl-sci↗

arXiv2018

Amplitude mode in the planar triangular antiferromagnet Na$_{0.9}$MnO$_2$

Amplitude modes arising from symmetry breaking in materials are of broad interest in condensed matter physics. These modes reflect an oscillation in the amplitude of a complex order parameter, yet are typically unstable and decay into oscillations of the order parameter's phase. This renders stable amplitude modes rare, and exotic effects in quantum antiferromagnets have historically provided a realm for their detection. Here we report an alternate route to realizing amplitude modes in magnetic materials by demonstrating that an antiferromagnet on a two-dimensional anisotropic triangular lattice ($α$-Na$_{0.9}$MnO$_2$) exhibits a long-lived, coherent oscillation of its staggered magnetization field. Our results show that geometric frustration of Heisenberg spins with uniaxial single-ion anisotropy can renormalize the interactions of a dense two-dimensional network of moments into largely decoupled, one-dimensional chains that manifest a longitudinally polarized bound state. This bound state is driven by the Ising-like anisotropy inherent to the Mn$^{3+}$ ions of this compound.

cond-mat.str-el↗

arXiv2018

Thermal evolution of quasi-1D spin correlations within the anisotropic triangular lattice of $α$-NaMnO$_2$

Magnetic order on the spatially anisotropic triangular lattice of $α$-NaMnO$_2$ is studied via neutron diffraction measurements. The transition into a commensurate, collinear antiferromagnetic ground state with $\mathbf{k}=(0.5, 0.5, 0)$ was found to occur below $T_\mathrm{N}=22$ K. Above this temperature, the transition is preceded by the formation of a coexisting, short-range ordered, incommensurate state below $T_\mathrm{IC}=45$ K whose two dimensional propagation vector evolves toward $\mathbf{k}=(0.5, 0.5)$ as the temperature approaches $T_{\mathrm{N}}$. At high temperatures ($T>T_\mathrm{IC}$), quasielastic scattering reveals one-dimensional spin correlations along the nearest neighbor Mn-Mn "chain direction" of the MnO$_6$ planes. Our data are consistent with the predictions of a mean field model of Ising-like spins on an anisotropic triangular lattice, as well as the predominantly one-dimensional Heisenberg spin Hamiltonian reported for this material.

cond-mat.str-el↗

arXiv2018

Disorder induced power-law gaps in an insulator-metal Mott transition

A correlated material in the vicinity of an insulator-metal transition (IMT) exhibits rich phenomenology and variety of interesting phases. A common avenue to induce IMTs in Mott insulators is doping, which inevitably leads to disorder. While disorder is well known to create electronic inhomogeneity, recent theoretical studies have indicated that it may play an unexpected and much more profound role in controlling the properties of Mott systems. Theory predicts that disorder might play a role in driving a Mott insulator across an IMT, with the emergent metallic state hosting a power law suppression of the density of states (with exponent close to 1; V-shaped gap) centered at the Fermi energy. Such V-shaped gaps have been observed in Mott systems but their origins are as yet unknown. To investigate this, we use scanning tunneling microscopy and spectroscopy to study isovalent Ru substitutions in Sr$_3$(Ir$_{1-x}$Ru$_x$)$_2$O$_7$ which drives the system into an antiferromagnetic, metallic state. Our experiments reveal that many core features of the IMT such as power law density of states, pinning of the Fermi energy with increasing disorder, and persistence of antiferromagnetism can be understood as universal features of a disordered Mott system near an IMT and suggest that V-shaped gaps may be an inevitable consequence of disorder in doped Mott insulators.

cond-mat.str-el↗

arXiv2018

Sr$_3$Ir$_2$O$_7$F$_2$: Topochemical conversion of a relativistic Mott state into a spin-orbit driven band insulator

The topochemical transformation of single crystals of Sr$_3$Ir$_2$O$_7$ into Sr$_3$Ir$_2$O$_7$F$_2$ is reported via fluorine insertion. Characterization of the newly formed Sr$_3$Ir$_2$O$_7$F$_2$ phase shows a nearly complete oxidation of Ir$^{4+}$ cations into Ir$^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled J$_{eff}=1/2$ band into a nonmagnetic $J=0$ band insulator. First principles calculations reveal a remarkably flat insertion energy that locally drives the fluorination process to completion. Band structure calculations support the formation of a band insulator whose charge gap relies on the strong spin-orbit coupling inherent to the Ir metal ions of this compound.

cond-mat.str-el↗

arXiv2018

Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3

Bismuth selenide (Bi2Se3) is a prototypical three-dimensional topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements in a thick exfoliated nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3-1.2 eV). Theoretical modeling of the reflectivity spectral lineshapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding of how such states interact dynamically with the topologically protected Dirac surface states.

cond-mat.mes-hall↗

arXiv2018

Electron doping in $\text{Sr}_3\text{Ir}_2\text{O}_7$: collapse of band gap and magnetic order

The electron-doping-driven collapse of the charge gap and staggered magnetization of the spin-orbit-assisted Mott insulator Sr$_{3}$Ir$_{2}$O$_{7}$ is explored via first-principles computational methods. In the antiferromagnetic phase, the gap and magnetization are observed to decrease slowly with increasing doping, with an abrupt collapse of both the gap and the magnetization at an electron concentration corresponding to 4.8\% substitution of Sr with La, in excellent agreement with experiment. Additionally, we describe the structural effects of electron doping in Sr$_{3}$Ir$_{2}$O$_{7}$ via a competition between the steric effect from smaller La atoms substituted within the lattice and the dominant doping-driven deformation-potential effect. Curiously, our first-principles calculations fail to capture the low-temperature structural distortion reported in the low-gap phase of Sr$_{3}$Ir$_{2}$O$_{7}$, supporting the notion that this distortion arises as a secondary manifestation of an unconventional electronic order parameter in this material.

cond-mat.str-el↗

arXiv2018

Magnetoentropic signatures of skyrmionic phase behavior in FeGe

We demonstrate that magnetocaloric measurements can rapidly reveal details of the phase diagrams of high-temperature skyrmion hosts, concurrently yielding quantitative latent heats of the field-driven magnetic phase transitions. Our approach addresses an outstanding issue in the phase diagram of the skyrmion host FeGe by showing that dc magnetic anomalies can be explained in terms of entropic signatures consistent with a phase diagram containing a single pocket of skyrmionic order and a Brazovskii transition.

cond-mat.mtrl-sci↗

arXiv2017

Doping driven structural distortion in the bilayer iridate (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$

Neutron single crystal diffraction and rotational anisotropy optical second harmonic generation data are presented resolving the nature of the structural distortion realized in electron-doped (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$ with $x=0.035$ and $x=0.071$. Once electrons are introduced into the bilayer spin-orbit assisted Mott insulator Sr$_3$Ir$_2$O$_7$, previous studies have identified the appearance of a low temperature structural distortion and have suggested the presence of a competing electronic instability in the phase diagram of this material. Our measurements resolve a lowering of the structural symmetry from monoclinic $C2/c$ to monoclinic $P2_1/c$ and the creation of two unique Ir sites within the chemical unit cell as the lattice distorts below a critical temperature $T_S$. Details regarding the modifications to oxygen octahedral rotations and tilting through the transition are discussed as well as the evolution of the low temperature distorted lattice as a function of carrier substitution.

cond-mat.str-el↗

arXiv2016

Floating zone growth of α-Na$_{0.90}$MnO$_2$ single crystals

Single crystal growth of α-Na$_x$MnO$_2$ (x = 0.90) is reported via the floating zone technique. The conditions required for stable growth and intergrowth-free crystals are described along with the results of trials under alternate growth atmospheres. Chemical and structural characterizations of the resulting α-Na$_{0.90}$MnO$_2$ crystals are performed using ICP-AES, NMR, XANES, XPS, and neutron diffraction measurements. As a layered transition metal oxide with large ionic mobility and strong correlation effects, α-Na$_x$MnO$_2$ is of interest to many communities, and the implications of large volume, high purity, single crystal growth are discussed.

cond-mat.str-el↗

arXiv2016

Structural evolution and electronic properties of (Sr1-xCax)2-yIrO4+z spin-orbit assisted insulators

The effect of isoelectronic substitution within single crystals of (Sr1-xCax)2-yIrO4+z is explored. The nominal n=1 Ruddlesden-Popper phase with y=0, z=0 remains stable from x=0 until x=0.11, where the antiferromagnet spin-orbit Mott insulating state persists. An increase in the saturated moment is observed with increasing Ca-substitution, suggesting a modified coupling of the in-plane moments relative to the in-plane rotation of IrO6 octahedra. Beyond x=0.11, the x=1/4, y=0, z=1/2 structural phase Sr3CaIr2O9, consisting of a three-dimensional network of corner sharing octahedra, begins to intermix and eventually nucleates phase pure crystals at higher starting Ca-content. An insulating, nonmagnetic ground state is observed in this phase attributable to the J=0 state and is consistent with a recent powder study. At higher Ca-concentrations beyond x = 0.75, crystals begin to stabilize in the y=1/3, z=0 quasi one-dimensional Ca5Ir3O12 structure. The low temperature transport in this chain-based structure is well described via variable range hopping, and an antiferromagnetic ordering transition appears below T$_N=9$ K. Our data provide a detailed mapping of the electronic and structural properties accessible as the structural framework of the canonical spin orbit Mott insulator Sr2IrO4 is destabilized via isovalent chemical substitution.

cond-mat.mtrl-sci↗

arXiv2016

Disordered dimer state in electron-doped Sr$_{3}$Ir$_{2}$O$_{7}$

Spin excitations are explored in the electron-doped spin-orbit Mott insulator (Sr$_{1-x}$La$_{x}$)$_3$Ir$_2$O$_7$. As this bilayer square lattice system is doped into the metallic regime, long-range antiferromagnetism vanishes, yet a spectrum of gapped spin excitation remains. Excitation lifetimes are strongly damped with increasing carrier concentration, and the energy integrated spectral weight becomes nearly momentum independent as static spin order is suppressed. Local magnetic moments, absent in the parent system, grow in metallic samples and approach values consistent with one $J=\frac{1}{2}$ impurity per electron doped. Our combined data suggest that the magnetic spectra of metallic (Sr$_{1-x}$La$_{x}$)$_3$Ir$_2$O$_7$ are best described by excitations out of a disordered dimer state.

cond-mat.str-el↗

arXiv2016

Interface-driven ferromagnetism within the quantum wells of a rare earth titanate superlattice

Here we present polarized neutron reflectometry measurements exploring thin film heterostructures comprised of a strongly correlated Mott state, GdTiO$_3$, embedded with SrTiO$_3$ quantum wells. Our results reveal that the net ferromagnetism inherent to the Mott GdTiO$_3$ matrix propagates into the nominally nonmagnetic SrTiO$_3$ quantum wells and tracks the magnetic order parameter of the host Mott insulating matrix. Beyond a well thickness of 5 SrO layers, the magnetic moment within the wells is dramatically suppressed, suggesting that quenched well magnetism comprises the likely origin of quantum critical magnetotransport in this thin film architecture. Our data demonstrate that the interplay between proximate exchange fields and polarity induced carrier densities can stabilize extended magnetic states within SrTiO$_3$ quantum wells.

cond-mat.str-el↗