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1.
Phys Rev Lett ; 125(8): 087202, 2020 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-32909791

RESUMO

We used a combination of polarized Raman spectroscopy and spin wave calculations to study magnetic excitations in the strong spin-orbit-coupled bilayer perovskite antiferromagnet Sr_{3}Ir_{2}O_{7}. We observed two broad Raman features at ∼800 and ∼1400 cm^{-1} arising from magnetic excitations. Unconventionally, the ∼800 cm^{-1} feature is fully symmetric (A_{1g}) with respect to the underlying tetragonal (D_{4h}) crystal lattice which, together with its broad line shape, definitively rules out the possibility of a single magnon excitation as its origin. In contrast, the ∼1400 cm^{-1} feature shows up in both the A_{1g} and B_{2g} channels. From spin wave and two-magnon scattering cross-section calculations of a tetragonal bilayer antiferromagnet, we identified the ∼800 cm^{-1} (1400 cm^{-1}) feature as two-magnon excitations with pairs of magnons from the zone-center Γ point (zone-boundary van Hove singularity X point). We further found that this zone-center two-magnon scattering is unique to bilayer perovskite magnets which host an optical branch in addition to the acoustic branch, as compared to their single layer counterparts. This zone-center two-magnon mode is distinct in symmetry from the time-reversal symmetry broken "spin wave gap" and "phase mode" proposed to explain the ∼92 meV (742 cm^{-1}) gap in resonant inelastic x-ray spectroscopy magnetic excitation spectra of Sr_{3}Ir_{2}O_{7}.

2.
Sci Adv ; 6(31): eabb6003, 2020 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-32789181

RESUMO

The anomalous Hall effect (AHE) is one of the most fundamental phenomena in physics. In the highly conductive regime, ferromagnetic metals have been the focus of past research. Here, we report a giant extrinsic AHE in KV3Sb5, an exfoliable, highly conductive semimetal with Dirac quasiparticles and a vanadium Kagome net. Even without report of long range magnetic order, the anomalous Hall conductivity reaches 15,507 Ω-1 cm-1 with an anomalous Hall ratio of ≈ 1.8%; an order of magnitude larger than Fe. Defying theoretical expectations, KV3Sb5 shows enhanced skew scattering that scales quadratically, not linearly, with the longitudinal conductivity, possibly arising from the combination of highly conductive Dirac quasiparticles with a frustrated magnetic sublattice. This allows the possibility of reaching an anomalous Hall angle of 90° in metals. This observation raises fundamental questions about AHEs and opens new frontiers for AHE and spin Hall effect exploration, particularly in metallic frustrated magnets.

3.
Nat Commun ; 11(1): 3076, 2020 Jun 17.
Artigo em Inglês | MEDLINE | ID: mdl-32555246

RESUMO

The physics of weak itinerant ferromagnets is challenging due to their small magnetic moments and the ambiguous role of local interactions governing their electronic properties, many of which violate Fermi-liquid theory. While magnetic fluctuations play an important role in the materials' unusual electronic states, the nature of these fluctuations and the paradigms through which they arise remain debated. Here we use inelastic neutron scattering to study magnetic fluctuations in the canonical weak itinerant ferromagnet MnSi. Data reveal that short-wavelength magnons continue to propagate until a mode crossing predicted for strongly interacting quasiparticles is reached, and the local susceptibility peaks at a coherence energy predicted for a correlated Hund metal by first-principles many-body theory. Scattering between electrons and orbital and spin fluctuations in MnSi can be understood at the local level to generate its non-Fermi liquid character. These results provide crucial insight into the role of interorbital Hund's exchange within the broader class of enigmatic multiband itinerant, weak ferromagnets.

4.
Phys Rev Lett ; 124(19): 197203, 2020 May 15.
Artigo em Inglês | MEDLINE | ID: mdl-32469556

RESUMO

Here we report on the formation of a three-magnon bound state in the quasi-one-dimensional antiferromagnet α-NaMnO_{2}, where the single-ion, uniaxial anisotropy inherent to the Mn^{3+} ions in this material provides a binding mechanism capable of stabilizing higher order magnon bound states. While such states have long remained elusive in studies of antiferromagnetic chains, neutron scattering data presented here demonstrate that higher order n>2 composite magnons exist, and, specifically, that a weak three-magnon bound state is detected below the antiferromagnetic ordering transition of NaMnO_{2}. We corroborate our findings with exact numerical simulations of a one-dimensional Heisenberg chain with easy-axis anisotropy using matrix-product state techniques, finding a good quantitative agreement with the experiment. These results establish α-NaMnO_{2} as a unique platform for exploring the dynamics of composite magnon states inherent to a classical antiferromagnetic spin chain with Ising-like single ion anisotropy.

5.
Artigo em Inglês | MEDLINE | ID: mdl-32251548

RESUMO

There has been a great deal of recent interest in extended compounds containing Ru3+ and Ru4+ in light of their range of unusual physical properties. Many of these properties are displayed in compounds with the perovskite and related structures. Here we report an array of structurally diverse hybrid ruthenium halide perovskites and related compounds: MA2 RuX6 (X=Cl or Br), MA2 MRuX6 (M=Na, K or Ag; X=Cl or Br) and MA3 Ru2 X9 (X=Br) based upon the use of methylammonium (MA=CH3 NH3 + ) on the perovskite A site. The compounds MA2 RuX6 with Ru4+ crystallize in the trigonal space group R 3 ‾ m and can be described as vacancy-ordered double-perovskites. The ordered compounds MA2 MRuX6 with M+ and Ru3+ crystallize in a structure related to BaNiO3 with alternating MX6 and RuX6 face-shared octahedra forming linear chains in the trigonal P 3 ‾ m space group. The compound MA3 Ru2 Br9 crystallizes in the orthorhombic Cmcm space group and displays pairs of face-sharing octahedra forming isolated Ru2 Br9 moieties with very short Ru-Ru contacts of 2.789 Å. The structural details, including the role of hydrogen bonding and dimensionality, as well as the optical and magnetic properties of these compounds are described. The magnetic behavior of all three classes of compounds is influenced by spin-orbit coupling and their temperature-dependent behavior has been compared with the predictions of the appropriate Kotani models.

6.
Phys Rev Lett ; 122(15): 157201, 2019 Apr 19.
Artigo em Inglês | MEDLINE | ID: mdl-31050510

RESUMO

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 end points. 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}.

7.
Rev Sci Instrum ; 90(4): 043906, 2019 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-31043029

RESUMO

The floating zone technique is a well-established single crystal growth method in materials research, which is 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.

8.
J Phys Condens Matter ; 31(24): 244003, 2019 Jun 19.
Artigo em Inglês | MEDLINE | ID: mdl-30861499

RESUMO

The breakdown of [Formula: see text] antiferromagnetism in the limit of strong disorder is studied in Sr3(Ir1-x Mn x )2O7. Upon Mn-substitution, antiferromagnetic ordering of the Ir cations becomes increasingly two-dimensional, resulting in the complete suppression of long-range Ir magnetic order above [Formula: see text]. Long-range antiferromagnetism however persists on the Mn sites to higher Mn concentrations (x > 0.25) and is necessarily mediated via a random network of majority Ir sites. Our data suggest a shift in the Mn valence from Mn4+ to Mn3+ at intermediate doping levels, which in turn generates nonmagnetic Ir5+ sites and suppresses long-range order within the Ir network. The collapse of long-range [Formula: see text] antiferromagnetism and the survival of percolating antiferromagnetic order on Mn-sites demonstrates a complex 3d-5d exchange process that surprisingly enables minority Mn spins to order far below the conventional percolation threshold for a bilayer square lattice.

9.
Proc Natl Acad Sci U S A ; 115(44): 11198-11202, 2018 10 30.
Artigo em Inglês | MEDLINE | ID: mdl-30322914

RESUMO

A correlated material in the vicinity of an insulator-metal transition (IMT) exhibits rich phenomenology and a 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 Sr3(Ir1-xRux)2O7 (0 ≤ x ≤ 0.5) which drive 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.

10.
Nano Lett ; 18(9): 5875-5884, 2018 09 12.
Artigo em Inglês | MEDLINE | ID: mdl-30106301

RESUMO

Bismuth selenide (Bi2Se3) is a prototypical 3D 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 on a single 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 to 1.2 eV). Theoretical modeling of the reflectivity spectral line shapes 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 how such states interact dynamically with the topologically protected Dirac surface states.

11.
Nat Commun ; 9(1): 2188, 2018 06 05.
Artigo em Inglês | MEDLINE | ID: mdl-29872040

RESUMO

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-Na0.9MnO2) 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 Mn3+ ions of this compound.

12.
ACS Appl Mater Interfaces ; 10(8): 7208-7213, 2018 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-29457889

RESUMO

Rapid preparation utilizing assisted microwave heating permits significantly shorter preparation times for magnetocaloric compounds in the (Mn,Fe)2(P,Si) family, specifically samples of (Mn,Fe)2-δP0.5Si0.5 with starting compositions of δ = 0, 0.06, and 0.12. To fully understand the effects of processing and composition changes on structure and properties, these materials are characterized using synchrotron powder diffraction, neutron powder diffraction, electron microprobe analysis (EMPA), X-ray fluorescence (XRF), and magnetic measurements. The diffraction analysis reveals that increasing δ results in decreasing amounts of the common Heusler (Mn,Fe)3Si secondary phase. EMPA shows (Mn,Fe)2(P,Si) in all three samples to be Mn and P rich, whereas XRF demonstrates that the bulk material is Mn rich yet P deficient. Increasing δ brings the Mn/Fe and P/Si ratios closer to their starting values. Measurements of magnetic properties show an increase in saturation magnetization and ordering temperature with increasing δ, consistent with the increase in Fe and Si contents. Increasing δ also results in a decrease in thermal hysteresis and an increase in magnetic entropy change, the latter reaching values close to what have been previously reported on samples that take much longer to prepare.

13.
Nat Commun ; 9(1): 103, 2018 01 09.
Artigo em Inglês | MEDLINE | ID: mdl-29317642

RESUMO

Materials that exhibit both strong spin-orbit coupling and electron correlation effects are predicted to host numerous new electronic states. One prominent example is the Jeff = 1/2 Mott state in Sr2IrO4, where introducing carriers is predicted to manifest high temperature superconductivity analogous to the S = 1/2 Mott state of La2CuO4. While bulk superconductivity currently remains elusive, anomalous quasiparticle behaviors paralleling those in the cuprates such as pseudogap formation and the formation of a d-wave gap are observed upon electron-doping Sr2IrO4. Here we establish a magnetic parallel between electron-doped Sr2IrO4 and hole-doped La2CuO4 by unveiling a spin density wave state in electron-doped Sr2IrO4. Our magnetic resonant X-ray scattering data reveal the presence of an incommensurate magnetic state reminiscent of the diagonal spin density wave state observed in the monolayer cuprate (La1-xSr x )2CuO4. This link supports the conjecture that the quenched Mott phases in electron-doped Sr2IrO4 and hole-doped La2CuO4 support common competing electronic phases.

14.
Am J Med Qual ; 32(1): 27-33, 2017.
Artigo em Inglês | MEDLINE | ID: mdl-26635330

RESUMO

This study examines whether implementing a resident shift work schedule (RSWS) alone or combined with a hospitalist-led model system (HMS/RSWS) affects patient care outcomes or costs at a pediatric tertiary care teaching hospital. A retrospective sample compared pre- and postintervention groups for the most common primary discharge diagnoses, including asthma and cellulitis (RSWS intervention) and inflammatory bowel disease and diabetic ketoacidosis (HMS/RSWS intervention). Outcome variables included length of stay, number of subspecialty consultations, and hospitalization charges. For the RSWS intervention, the preintervention (n = 107) and postintervention (n = 92) groups showed no difference in any of the outcome variables. For the HMS/RSWS intervention, the preintervention (n = 98) and postintervention (n = 69) groups did not differ in demographics or length of stay. However, subspecialty consultations increased significantly during postintervention from 0.83 to 1.52 consults/hospitalization ( P < .01) without significantly increasing hospitalization charges. Neither the RSWS nor HMS/RSWS intervention affected patient care outcomes at a pediatric tertiary care teaching hospital.


Assuntos
Médicos Hospitalares/organização & administração , Hospitais de Ensino/organização & administração , Internato e Residência/organização & administração , Tempo de Internação/estatística & dados numéricos , Admissão e Escalonamento de Pessoal/organização & administração , Asma/terapia , Celulite (Flegmão)/terapia , Criança , Pré-Escolar , Cetoacidose Diabética/terapia , Preços Hospitalares , Médicos Hospitalares/economia , Hospitais Pediátricos/organização & administração , Hospitais de Ensino/economia , Hospitais de Ensino/estatística & dados numéricos , Humanos , Lactente , Doenças Inflamatórias Intestinais/terapia , Internato e Residência/economia , Admissão e Escalonamento de Pessoal/economia , Admissão e Escalonamento de Pessoal/estatística & dados numéricos , Qualidade da Assistência à Saúde/organização & administração , Estudos Retrospectivos , Especialização
15.
Phys Rev Lett ; 117(3): 037205, 2016 Jul 15.
Artigo em Inglês | MEDLINE | ID: mdl-27472135

RESUMO

Here we present polarized neutron reflectometry measurements exploring thin film heterostructures composed 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.

16.
J Phys Condens Matter ; 27(37): 375403, 2015 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-26328745

RESUMO

PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated. The results indicate that the virtual crystal model is overall a good assumption for phonon frequencies and group velocities despite the softening of transverse acoustic phonon modes along [1 1 1] direction, while the treatment of lifetimes warrants caution. In addition, phonons remain a good description of vibrational modes in PbTe1-x Se x alloys.


Assuntos
Ligas/química , Fônons , Difração de Raios X , Chumbo/química , Teoria Quântica , Selênio/química , Telúrio/química
18.
Phys Rev Lett ; 114(25): 257203, 2015 Jun 26.
Artigo em Inglês | MEDLINE | ID: mdl-26197142

RESUMO

The electronic phase diagram of the weak spin-orbit Mott insulator (Sr(1-x)La(x))(3)Ir(2)O(7) is determined via an exhaustive experimental study. Upon doping electrons via La substitution, an immediate collapse in resistivity occurs along with a narrow regime of nanoscale phase separation comprised of antiferromagnetic, insulating regions and paramagnetic, metallic puddles persisting until x≈0.04. Continued electron doping results in an abrupt, first-order phase boundary where the Néel state is suppressed and a homogenous, correlated, metallic state appears with an enhanced spin susceptibility and local moments. As the metallic state is stabilized, a weak structural distortion develops and suggests a competing instability with the parent spin-orbit Mott state.

19.
Phys Rev Lett ; 113(24): 247601, 2014 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-25541804

RESUMO

The magnetic ground state of the J(eff)=1/2 hyperkagome lattice in Na4Ir3O8 is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen state comprised of quasistatic moments develops below a characteristic temperature of T(F)=6 K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasistatic, short-range spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Our data demonstrate that an inhomogeneous magnetic ground state arises in Na4Ir3O8 driven either by disorder inherent to the creation of the hyperkagome lattice itself or stabilized via quantum fluctuations.

20.
Sci Rep ; 4: 4120, 2014 Feb 18.
Artigo em Inglês | MEDLINE | ID: mdl-24844399

RESUMO

The interplay between magnetism and crystal structures in three CaFe2As2 samples is studied. For the nonmagnetic quenched crystals, different crystalline domains with varying lattice parameters are found, and three phases (orthorhombic, tetragonal, and collapsed tetragonal) coexist between TS=95 K and 45 K. Annealing of the quenched crystals at 350°C leads to a strain relief through a large (~1.3%) expansion of the c-parameter and a small (~0.2%) contraction of the a-parameter, and to local ~0.2 Šdisplacements at the atomic-level. This annealing procedure results in the most homogeneous crystals for which the antiferromagnetic and orthorhombic phase transitions occur at TN/TS=168(1) K. In the 700°C-annealed crystal, an intermediate strain regime takes place, with tetragonal and orthorhombic structural phases coexisting between 80 to 120 K. The origin of such strong shifts in the transition temperatures are tied to structural parameters. Importantly, with annealing, an increase in the Fe-As length leads to more localized Fe electrons and higher local magnetic moments on Fe ions. Synergistic contribution of other structural parameters, including a decrease in the Fe-Fe distance, and a dramatic increase of the c-parameter, which enhances the Fermi surface nesting in CaFe2As2, are also discussed.

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