Showing papers by "Robert J. Birgeneau published in 2022"

Discovery of charge density wave in a kagome lattice antiferromagnet

Abstract: A hallmark of strongly correlated quantum materials is the rich phase diagram resulting from competing and intertwined phases with nearly degenerate ground-state energies1,2. A well-known example is the copper oxides, in which a charge density wave (CDW) is ordered well above and strongly coupled to the magnetic order to form spin-charge-separated stripes that compete with superconductivity1,2. Recently, such rich phase diagrams have also been shown in correlated topological materials. In 2D kagome lattice metals consisting of corner-sharing triangles, the geometry of the lattice can produce flat bands with localized electrons3,4, non-trivial topology5–7, chiral magnetic order8,9, superconductivity and CDW order10–15. Although CDW has been found in weakly electron-correlated non-magnetic AV3Sb5 (A = K, Rb, Cs)10–15, it has not yet been observed in correlated magnetic-ordered kagome lattice metals4,16–21. Here we report the discovery of CDW in the antiferromagnetic (AFM) ordered phase of kagome lattice FeGe (refs. 16–19). The CDW in FeGe occurs at wavevectors identical to that of AV3Sb5 (refs. 10–15), enhances the AFM ordered moment and induces an emergent anomalous Hall effect22,23. Our findings suggest that CDW in FeGe arises from the combination of electron-correlations-driven AFM order and van Hove singularities (vHSs)-driven instability possibly associated with a chiral flux phase24–28, in stark contrast to strongly correlated copper oxides1,2 and nickelates29–31, in which the CDW precedes or accompanies the magnetic order. Analysis of the antiferromagnetic ordered phase of kagome lattice FeGe suggests that charge density wave is the result of a combination of electronic-correlations-driven antiferromagnetic order and instability driven by van Hove singularities.

A room temperature polar magnetic metal

Abstract: The emergence of long-range magnetic order in noncentrosymmetric compounds has stimulated interest in the possibility of exotic spin transport phenomena and topologically protected spin textures for applications in next-generation spintronics. Polar magnets, with broken symmetries of spatial inversion and time reversal, usually host chiral spin textures. This work reports on a wurtzite-structure polar magnetic metal, identified as AA′-stacked (Fe0.5Co0.5)5GeTe2, which exhibits a Néel-type skyrmion lattice as well as a Rashba-Edelstein effect at room temperature. Atomic resolution imaging of the structure reveals a structural transition as a function of Co-substitution, leading to the emergence of the polar phase at 50% Co. This discovery reveals an unprecedented layered polar magnetic system for investigating intriguing spin topologies, and it ushers in a promising new framework for spintronics.Received 16 October 2021Accepted 8 March 2022DOI:https://doi.org/10.1103/PhysRevMaterials.6.044403©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMagnetotransportSkyrmionsSpintronicsPhysical SystemsTransition metal dichalcogenidesCondensed Matter, Materials & Applied Physics

Correlation-driven electronic reconstruction in FeTe1−xSex

Abstract: Electronic correlation is of fundamental importance to high temperature superconductivity. Iron-based superconductors are believed to possess moderate correlation strength, which combined with their multi-orbital nature makes them a fascinating platform for the emergence of exotic phenomena. A particularly striking form is the emergence of an orbital selective Mott phase, where the localization of a subset of orbitals leads to a drastically reconstructed Fermi surface. Here, we report spectroscopic evidence of the reorganization of the Fermi surface from FeSe to FeTe as Se is substituted by Te. We uncover a particularly transparent way to visualize the localization of the $d_{xy}$ electron orbital through the suppression of its hybridization with the more coherent $d$ electron orbitals, which leads to a redistribution of the orbital-dependent spectral weight near the Fermi level. These noteworthy features of the Fermi surface are accompanied by a divergent behavior of a band renormalization in the $d_{xy}$ orbital. All of our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase towards an orbital-selective Mott phase in FeTe$_{1-x}$Se$_x$ as Se concentration is reduced.

Magnetism and charge density wave order in kagome FeGe

Abstract: Electron correlations often lead to emergent orders in quantum materials, and one example is the kagome lattice materials where topological states exist in the presence of strong correlations between electrons. This arises from the features of the electronic band structure that are associated with the kagome lattice geometry: flat bands induced by destructive interference of the electronic wavefunctions, topological Dirac crossings and a pair of van Hove singularities. Various correlated electronic phases have been discovered in kagome lattice materials, including magnetism, charge density waves, nematicity and superconductivity. Recently, a charge density wave was discovered in the magnetic kagome FeGe, providing a platform for understanding the interplay between charge order and magnetism in kagome materials. Here we observe all three electronic signatures of the kagome lattice in FeGe using angle-resolved photoemission spectroscopy. The presence of van Hove singularities near the Fermi level is driven by the underlying magnetic exchange splitting. Furthermore, we show spectral evidence for the charge density wave as gaps near the Fermi level. Our observations point to the magnetic interaction-driven band modification resulting in the formation of the charge density wave and indicate an intertwined connection between the emergent magnetism and charge order in this moderately correlated kagome metal. The observation of band structure features typical of the kagome lattice in FeGe suggests that an interplay of magnetism and electronic correlations determines the physics of this material.

Uniaxial ferromagnetism in the kagome metal TbV${_6}$Sn${_6}$

Abstract: The synthesis and characterization of the vanadium-based kagome metal TbV 6 Sn 6 is presented. X-ray measurements confirm this material forms with the same crystal structure type as the recently investigated kagome metals GdV 6 Sn 6 and YV 6 Sn 6 , with space group symmetry P6/mmm. A signature of a phase transition at 4.1K is observed in heat capacity, resistivity, and magnetic susceptibility measurements, and both resistivity and magnetization measurements exhibit hysteresis in magnetic field. Furthermore, a strikingly large anisotropy in the magnetic susceptibility was observed, with the c-axis susceptibility nearly 100 times the ab plane susceptibility at 2K. This is highly suggestive of uniaxial ferromagnetism, and the large size of 9.4 µ b /f.u. indicates the Tb 3+ 4 f electronic moments cooperatively align perpendicular to the V kagome lattice plane. The entropy at the phase transition is nearly Rln(2), indicating that the CEF ground state of the Tb 3+ ion is a doublet, and therefore the sublattice of 4 f electrons in this material can be shown to map at low temperatures to the Ising model in a D 6 h symmetry environment. Hall measurements at temperatures from 300K to 1.7K can be described by two-band carrier transport at temperatures below around 150K, with a large increase in both hole and electron mobilities, similar to YV 6 Sn 6 , and an anomalous Hall effect is seen below the ordering temperature. Angle-resolved photoemission measurements above the magnetic ordering temperature reveal typical kagome dispersions. Our study presents TbV 6 Sn 6 as an ideal system to study the interplay between Ising ferromagnetism and non-trivial electronic states emerging from a kagome lattice.

Highly Tunable Magnetic Phases in Transition-Metal Dichalcogenide <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Fe</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo stretchy="false">/</mml:mo><mml:mn>3</mml:mn><mml:mo>+</mml:mo>

Abstract: A change in magnetic moment configurations as a function of iron ratio in Fe${}_{1/3+\ensuremath{\delta}}$NbS${}_{2}$ underlies its current-induced resistance switching, a key insight for using this and similar materials in future spintronic devices.

Pervasive beyond Room-Temperature Ferromagnetism in a Doped van der Waals Magnet.

Abstract: The existence of long-range magnetic order in low-dimensional magnetic systems, such as the quasi-two-dimensional van der Waals (vdW) magnets, has attracted intensive studies of new physical phenomena. The vdW Fe_{N}GeTe_{2} (N=3, 4, 5; FGT) family is exceptional, owing to its vast tunability of magnetic properties. In particular, a ferromagnetic ordering temperature (T_{C}) above room temperature at N=5 (F5GT) is observed. Here, our study shows that, by nickel (Ni) substitution of iron in F5GT, a record high T_{C}=478(6) K is achieved. Importantly, pervasive, beyond room-temperature ferromagnetism exists in almost the entire doping range of the phase diagram of Ni-F5GT. We argue that this striking observation in Ni-F5GT can be possibly due to several contributing factors, including increased 3D magnetic couplings due to the structural alterations.

Nonsymmorphic symmetry-protected band crossings in a square-net metal PtPb4

Abstract: Abstract Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb 4 , arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb 4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries.

Uniaxial ferromagnetism in the kagome metal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>TbV</mml:mi><mml:mn>6</mml:mn></mml:msub><mml:msub><mml:mi>Sn</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract: The synthesis and characterization of the vanadium-based kagome metal TbV${_6}$Sn${_6}$ is presented. X-ray measurements confirm this material forms with the same crystal structure type as the recently investigated kagome metals GdV$_6$Sn$_6$ and YV$_6$Sn$_6$, with space group symmetry P6/mmm. A signature of a phase transition at 4.1K is observed in heat capacity, resistivity, and magnetic susceptibility measurements, and both resistivity and magnetization measurements exhibit hysteresis in magnetic field. Furthermore, a strikingly large anisotropy in the magnetic susceptibility was observed, with the c-axis susceptibility nearly 100 times the ab plane susceptibility at 2K. This is highly suggestive of uniaxial ferromagnetism, and the large size of 9.4$\mu_b$/f.u. indicates the Tb$^{3+}$ $4f$ electronic moments cooperatively align perpendicular to the V kagome lattice plane. The entropy at the phase transition is nearly Rln(2), indicating that the CEF ground state of the Tb$^{3+}$ ion is a doublet, and therefore the sublattice of $4f$ electrons in this material can be shown to map at low temperatures to the Ising model in a D$_{6h}$ symmetry environment. Hall measurements at temperatures from 300K to 1.7K can be described by two-band carrier transport at temperatures below around 150K, with a large increase in both hole and electron mobilities, similar to YV$_6$Sn$_6$, and an anomalous Hall effect is seen below the ordering temperature. Angle-resolved photoemission measurements above the magnetic ordering temperature reveal typical kagome dispersions. Our study presents TbV${_6}$Sn${_6}$ as an ideal system to study the interplay between Ising ferromagnetism and non-trivial electronic states emerging from a kagome lattice.

Nonsymmorphic symmetry-protected band crossings in a square-net metal PtPb4

Abstract: Abstract Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb 4 , arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb 4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries.

Gate-Tunable Transport in Quasi-One-Dimensional α-Bi4I4 Field Effect Transistors.

Abstract: Bi4I4 belongs to a novel family of quasi-one-dimensional (1D) topological insulators (TIs). While its β phase was demonstrated to be a prototypical weak TI, the α phase, long thought to be a trivial insulator, was recently predicted to be a rare higher order TI. Here, we report the first gate tunable transport together with evidence for unconventional band topology in exfoliated α-Bi4I4 field effect transistors. We observe a Dirac-like longitudinal resistance peak and a sign change in the Hall resistance; their temperature dependences suggest competing transport mechanisms: a hole-doped insulating bulk and one or more gate-tunable ambipolar boundary channels. Our combined transport, photoemission, and theoretical results indicate that the gate-tunable channels likely arise from novel gapped side surface states, two-dimensional (2D) TI in the bottommost layer, and/or helical hinge states of the upper layers. Markedly, a gate-tunable supercurrent is observed in an α-Bi4I4 Josephson junction, underscoring the potential of these boundary channels to mediate topological superconductivity.

Antiferromagnetic order in Co-doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Fe</mml:mi><mml:mn>5</mml:mn></mml:msub><mml:msub><mml:mi>GeTe</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> probed by resonant magnetic x-ray scattering

Abstract: The quasi-two-dimensional van der Waals magnet ${\mathrm{Fe}}_{5\ensuremath{-}\ensuremath{\delta}}{\mathrm{GeTe}}_{2}$ has emerged as a promising platform for electronic and spintronic functionalities at room temperature, owing to its large ferromagnetic ordering temperature ${T}_{\text{C}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 315 K. Interestingly, by cobalt (Co) substitution of iron in F5GT, $\text{i.e.,}\phantom{\rule{4pt}{0ex}}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{5\ensuremath{-}\ensuremath{\delta}}{\text{GeTe}}_{2}$ (Co-F5GT), not only can its magnetic transition temperature be further enhanced, but the magnetic and structural ground states can also be tuned. Specifically, an antiferromagnetic (AFM) order is induced beyond the Co doping level $x\ensuremath{\ge}0.4$. Here, we investigate the magnetic properties of a Co-F5GT single crystal at $x=0.45(1)$, by utilizing the element-specific, resonant magnetic x-ray scattering technique. Our study reveals an A-type, Ising-like AFM ground state, with a transition temperature ${T}_{\text{N}}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 340 K. In addition, our work unveils an important contribution from Co magnetic moments to the magnetic order. The application of the in-plane magnetic fields gradually polarizes the spin moments along the field direction, but without inducing incommensurate spin texture(s).

Weyl nodal ring states and Landau quantization with very large magnetoresistance in square-net magnet EuGa$_4$

Abstract: Magnetic topological semimetals (TSMs) allow for an effective control of the topological electronic states by tuning the spin configuration, and therefore are promising materials for next-generation electronic and spintronic applications. Of magnetic TSMs, Weyl nodal-line (NL) semimetals likely have the most tunability, and yet they are the least experimentally studied so far due to the scarcity of material candidates. Here, using a combination of angle-resolved photoemission spectroscopy and quantum oscillation measurements, together with density functional theory calculations, we identify the square-net compound EuGa 4 as a new magnetic Weyl NL semimetal, in which the line nodes form closed rings in the vicinity of the Fermi level. Remarkably, the Weyl nodal ring states show distinct Landau quantization with spin splitting on application of a magnetic field, resulting in prominent magnetotransport properties. At 2 kelvin in a field of 14 tesla, the transverse magnetoresistance of EuGa 4 exceeds 200,000%, which is more than two orders of magnitude larger than that of other known magnetic TSMs. High field magnetoresistance measurements indicate no saturation up to 40 T. Our results point to the realization of magnetic Weyl NL states in square-net materials, and opens new avenues for the design of magnetic TSMs with very large magnetoresistance.

Kramers nodal lines and Weyl fermions in SmAlSi

Abstract: Abstract Kramers nodal lines (KNLs) have recently been proposed theoretically as a special type of Weyl line degeneracy connecting time-reversal invariant momenta. KNLs are robust to spin orbit coupling and are inherent to all non-centrosymmetric achiral crystal structures, leading to unusual spin, magneto-electric, and optical properties. However, their existence in in real quantum materials has not been experimentally established. Here we gather the experimental evidence pointing at the presence of KNLs in SmAlSi, a non-centrosymmetric metal that develops incommensurate spin density wave order at low temperature. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and magneto-transport methods, we provide evidence suggesting the presence of KNLs, together with observing Weyl fermions under the broken inversion symmetry in the paramagnetic phase of SmAlSi. We discuss the nesting possibilities regarding the emergent magnetic orders in SmAlSi. Our results provide a solid basis of experimental observations for exploring correlated topology in SmAlSi

2 1 D ec 1 99 8 An X-Ray Induced Structural Transition in La 0

Abstract: We report a synchrotron x-ray scattering study of the magnetoresistive manganite La0.875Sr0.125MnO3. At low temperatures, this material undergoes an x-ray induced structural transition at which charge ordering of Mn and Mn ions characteristic to the low-temperature state of this compound is destroyed. The transition is persistent but the charge-ordered state can be restored by heating above the charge-ordering transition temperature and subsequently cooling. The charge-ordering diffraction peaks, which are broadened at all temperatures, broaden more upon x-ray irradiation, indicating the finite correlation length of the charge-ordered state. Together with the recent reports on x-ray induced transitions in Pr1−xCaxMnO3, our results demonstrate that the photoinduced structural change is a common property of the charge-ordered perovskite manganites.

Ordering of room-temperature magnetic skyrmions in a polar van der Waals magnet

Abstract: Control and understanding of ensembles of skyrmions is important for realization of future technologies. In particular, the order-disorder transition associated with the 2D lattice of magnetic skyrmions can have significant implications for transport and other dynamic functionalities. To date, skyrmion ensembles have been primarily studied in bulk crystals, or as isolated skyrmions in thin film devices. Here, we investigate the condensation of the skyrmion phase at room temperature and zero field in a polar, van der Waals magnet. We demonstrate that we can engineer an ordered skyrmion crystal through structural confinement on the μm scale, showing control over this order-disorder transition on scales relevant for device applications.

Thermal cycling induced alteration of the stacking order and spin-flip in the room temperature van der Waals magnet Fe$_5$GeTe$_2$

Abstract: The magnetic properties of the quasi-two-dimensional van der Waals magnet Fe$_{5-\delta}$GeTe$_2$ (F5GT), which has a high ferromagnetic ordering temperature $T_{\text{C}}$ $\sim$ 315 K, remains to be better understood. It has been demonstrated that the magnetization of F5GT is sensitive to both the Fe deficiency ${\delta}$ and the thermal cycling history. Here, we investigate the structural and magnetic properties of F5GT with a minimal Fe deficiency ($|{\delta}|$ $\le$ 0.1), utilizing combined x-ray and neutron scattering techniques. Our study reveals that the quenched F5GT single crystals experience an irreversible, first-order transition at $T_{\text{S}}$ $\sim$ 110 K upon first cooling, where the stacking order partly or entirely converts from ABC-stacking to AA-stacking order. Importantly, the magnetic properties, including the magnetic moment direction and the enhanced $T_{\text{C}}$ after the thermal cycling, are intimately related to the alteration of the stacking order. Our work highlights the significant influence of the lattice symmetry to the magnetism in F5GT.

Phonon softening and slowing down of charge-density-wave fluctuations in BaNi$_2$As$_2$

Abstract: BaNi$_2$As$_2$ is a nonmagnetic analogue of the iron pnictide superconductors, and in its tetragonal state exhibits an incommensurate charge-density-wave (IC-CDW) and a sizable elastoresistance. In this work, phonons in BaNi$_2$As$_2$ associated with the IC-CDW and uniform in-plane lattice distortions are investigated using high-resolution inelastic X-ray scattering. The in-plane transverse acoustic phonons reveal no softening at temperatures where the elastoresistance increases strongly, indicating the latter to be electronically driven. Systematic phonon measurements reveal the IC-CDW occurs in two stages upon cooling: underdamped phonons first soften to zero energy well above the IC-CDW ordering temperature, then the resulting quasielastic IC-CDW fluctuations gradually slow down and coalesce into the static IC-CDW order. A possible origin for our observations is the IC-CDW in tetragonal BaNi$_2$As$_2$ being uniaxial, which provides an additional Ising degree of freedom favorable for disordered IC-CDW modulations, and accounts for the elastoresistance through a weak coupling to the lattice.

Spectral Evidence for Unidirectional Charge Density Wave in Detwinned BaNi$_2$As$_2$

Abstract: The emergence of unconventional superconductivity in proximity to intertwined electronic orders is especially relevant in the case of iron-based superconductors. Such order consists of an electronic nematic order and a spin density wave in these systems. BaNi 2 As 2 , like its well-known iron-based analog BaFe 2 As 2 , also hosts a symmetry-breaking structural transition that is coupled to a unidirectional charge density wave (CDW), providing a novel platform to study intertwined orders. Here, through a systematic angle-resolved photoemission spectroscopy study combined with a detwinning B 1 g uniaxial strain, we identify distinct spectral evidence of band evolution due to the structural transition as well as CDW-induced band folding. In contrast to the nematicity and spin density wave in BaFe 2 As 2 , the structural and CDW order parameters in BaNi 2 As 2 are observed to be strongly coupled and do not separate in the presence of uniaxial strain. Our measurements point to a likely lattice origin of the CDW in BaNi 2 As 2 .

Effect of iron vacancies on magnetic order and spin dynamics of the spin ladder BaFe 2 − δ S 1 . 5 Se 1 . 5

Abstract: Quasi-one-dimensional iron chalcogenides possess various magnetic states depending on the lattice distortion, electronic correlations, and presence of defects. We present neutron diffraction and inelastic neutron scattering experiments on the spin ladder compound BaFe 2 − δ S 1 . 5 Se 1 . 5 with ∼ 6% iron vacancies. The data reveal that long-range magnetic order is absent, while the characteristic magnetic excitations that correspond to both the stripe-and block-type antiferromagnetic correlations are observed. First-principles calculations support the existence of both stripe- and block-type antiferromagnetic short-range orders in the experimental sample. The disappearance of long-range magnetic order may be due to the competition between these two magnetic orders, which is greatly enhanced for a certain concentration of iron vacancies, which we calculate to be about 6%, consistent with the measured iron vacancy concentration. Our results highlight how iron vacancies in the iron-based spin ladder system strongly influence the magnetic ground state.

Effect of iron vacancies on magnetic order and spin dynamics of the spin ladder <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">BaFe</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi mathva

Abstract: Quasi-one-dimensional iron chalcogenides possess various magnetic states depending on the lattice distortion, electronic correlations, and presence of defects. We present neutron diffraction and inelastic neutron scattering experiments on the spin ladder compound BaFe$_{2-\delta}$S$_{1.5}$Se$_{1.5}$ with $\sim$6% iron vacancies. The data reveal that long-range magnetic order is absent, while the characteristic magnetic excitations that correspond to both the stripe- and block-type antiferromagnetic correlations are observed. First-principles calculations support the existence of both stripe and block-type antiferromagnetic short-range order in the experimental sample. The disappearance of long-range magnetic order may be due to the competition between these two magnetic orders, which is greatly enhanced for a certain concentration of iron vacancies, which we calculate to be about 6%, consistent with the measured iron vacancy concentration. Our results highlight how iron vacancies in the iron-based spin ladder system strongly influence the magnetic ground state.

Effect of iron vacancies on the magnetic order and spin dynamics of the spin ladder

Abstract: Quasi-one-dimensional iron chalcogenides possess various magnetic states depending on the lattice distortion, electronic correlations, and presence of defects. We present neutron diffraction and inelastic neutron scattering experiments on the spin ladder compound BaFe 2 − δ S 1 . 5 Se 1 . 5 with ∼ 6% iron vacancies. The data reveal that long-range magnetic order is absent, while the characteristic magnetic excitations that correspond to both the stripe- and block-type antiferromagnetic correlations are observed. First-principles calculations support the existence of both stripe and block-type antiferromagnetic short-range order in the experimental sample. The disappearance of long-range magnetic order may be due to the competition between these two magnetic orders, which is greatly enhanced for a certain concentration of iron vacancies, which we calculate to be about 6%, consistent with the measured iron vacancy concentration. Our results highlight how iron vacancies in the iron-based spin ladder system strongly influence the magnetic ground state.

D ec 2 00 5 Resonant inelastic x-ray scattering study of overdoped La

Abstract: S. Wakimoto, 2, ∗ Young-June Kim, 3 Hyunkyung Kim, H. Zhang, T. Gog, and R. J. Birgeneau 5 1 Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7 Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan 3 Department of Physics, Brookhaven National Laboratory, Upton, New York 11973, USA 4 CMC-CAT, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA 5 Department of Physics, University of California, Berkeley, Berkeley, California 94720-7300, USA (Dated: March 23, 2022)

Antiferromagnetic order in Co-doped Fe$_5$GeTe$_2$ probed by resonant magnetic x-ray scattering

Abstract: The quasi-two-dimensional van der Waals magnet Fe 5 − δ GeTe 2 has emerged as a promising plat-form for electronic and spintronic functionalities at room temperature, owing to its large ferromagnetic ordering temperature T C ∼ 315 K. Interestingly, by cobalt (Co) substitution of iron in F5GT, i.e. (Fe 1 − x Co x ) 5 − δ GeTe 2 (Co-F5GT), not only can its magnetic transition temperature be further enhanced, but the magnetic and structural ground states can also be tuned. Specifically, an antiferromagnetic (AFM) order is induced beyond the Co doping level x ≥ 0 . 4. Here, we investigate the magnetic properties of a Co-F5GT single crystal at x = 0 . 45(1), by utilizing the element specific, resonant magnetic x-ray scattering technique. Our study reveals an A-type, Ising-like AFM ground state, with a transition temperature T N ∼ 340 K. In addition, our work unveils an important contribution from Co magnetic moments to the magnetic order. The application of the in-plane magnetic fields gradually polarize the spin moments along the field direction, but without inducing incommensurate spin texture(s).

Ju n 20 00 Static and dynamic spin correlations in the spin-glass phase of slightly-doped

Abstract: Neutron scattering experiments reveal that a diagonal spin modulation, which is a one-dimensional modulation rotated away by 45 from that in the superconducting phase, occurs universally across the insulating spin-glass phase in La2−xSrxCuO4 (0.02≤ x ≤0.055). This establishes an intimate relation between the magnetism and the transport properties in the high-temperature copper oxide superconductors. Furthermore, it is found that the charge density per unit length estimated using a charge stripe model is almost constant throughout the phase diagram, even when the modulation rotates away by 45 at the superconducting boundary. However, at the lowest values for x the density changes approaching 1 hole/Cu as in La2−xSrxNiO4. Magnetic excitation spectra suggest that magnetic correlations change from incommensurate to commensurate at ω ∼7 meV and T ∼70 K, indicating a characteristic energy for the incommensurate structure of 6-7 meV.

Fe b 20 01 Critical spin dynamics of the 2 D quantum Heisenberg antiferromagnets : Sr 2 CuO

Abstract: Y. J. Kim, R. J. Birgeneau, F. C. Chou, R. W. Erwin, and M. A. Kastner Department of Physics and Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (March 21, 2022)

Unravelling Temperature-Dependent Ordered Skyrmion Phases in Magnetic Layered Materials using Lorentz transmission Electron Microscopy

Abstract: Lorentz transmission electron microscopy (LTEM) is an exciting platform for the study of topological magnetic textures in materials, allowing imaging of magnetic domains and skyrmions under a variety of temperature and field conditions Co-doped Fe 5 GeTe 2 (FCGT) is a two-dimensional Van der Waals ferromagnet that exhibits room temperature, zero-field skyrmions when doped to precisely 50% Co concentration Recent studies have probed the temperature-magnetic field phase diagram of magnetic domain structures in this material with electrical transport measurements, but skyrmions tend to undergo complex 2D phase transitions that are best examined through real-space imaging techniques such as LTEM Fast, robust identification of skyrmions is required to study skyrmion ordering and dynamics of this type. identifying Bloch-type skyrmions using Laplacian of Gaussian (LoG) filters on raw LTEM images, but skyrmion identification a challenging The Néel skyrmions in FCGT further their non-circular geometry, very low contrast, proximity to regular magnetic domains and non-magnetic sample features, Here we present a

Effect of iron vacancies on the magnetic order and spin dynamics of the spin ladder BaFe$_{2-\delta}$S$_{1.5}$Se$_{1.5}$

Abstract: Quasi-one-dimensional iron chalcogenides possess various magnetic states depending on the lattice distortion, electronic correlations, and presence of defects. We present neutron diffraction and inelastic neutron scattering experiments on the spin ladder compound BaFe 2 − δ S 1 . 5 Se 1 . 5 with ∼ 6% iron vacancies. The data reveal that long-range magnetic order is absent, while the characteristic magnetic excitations that correspond to both the stripe- and block-type antiferromagnetic correlations are observed. First-principles calculations support the existence of both stripe and block-type antiferromagnetic short-range order in the experimental sample. The disappearance of long-range magnetic order may be due to the competition between these two magnetic orders, which is greatly enhanced for a certain concentration of iron vacancies, which we calculate to be about 6%, consistent with the measured iron vacancy concentration. Our results highlight how iron vacancies in the iron-based spin ladder system strongly influence the magnetic ground state.

Single-crystal growth and superconductivity in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>RbNi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Se</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract: We report the synthesis and characterization of ${\mathrm{RbNi}}_{2}{\mathrm{Se}}_{2}$, an analog of the iron chalcogenide superconductor ${\mathrm{Rb}}_{x}{\mathrm{Fe}}_{2}{\mathrm{Se}}_{2}$, via transport, angle-resolved photoemission spectroscopy, and density functional theory calculations. A superconducting transition at ${T}_{c}=1.20$ K is identified. In the normal state, ${\mathrm{RbNi}}_{2}{\mathrm{Se}}_{2}$ shows paramagnetic and Fermi-liquid behaviors. A large Sommerfeld coefficient yields an effective electron mass of ${m}^{*}\ensuremath{\approx}6{m}_{e}$. In the superconducting state, zero-field electronic specific-heat data ${C}_{es}$ can be described by a two-gap BCS model, indicating that ${\mathrm{RbNi}}_{2}{\mathrm{Se}}_{2}$ is a possible multigap superconductor. Our density functional theory calculations and angle-resolved photoemission spectroscopy measurements demonstrate that ${\mathrm{RbNi}}_{2}{\mathrm{Se}}_{2}$ exhibits relatively weak correlations and multiband characteristics, consistent with the multigap superconductivity.

S ep 2 00 1 Diagonal static spin correlation in the low temperature orthorhombic Pccn phase of La

Abstract: Elastic neutron-scattering measurements have been performed on La1.55Nd0.4Sr0.05CuO4, which exhibits a structural phase transition at Ts ∼ 60 K from the low temperature orthorhombic Bmab phase (labelled LTO1) to the low temperature orthorhombic Pccn phase (labelled LTO2). At low temperatures, well below Ts, elastic magnetic peaks are observed at the “diagonal” incommensurate (IC) positions (0, 1±0.055, 0), with the modulation direction only along the orthorhombic b-axis just as in Nd-free La1.95Sr0.05CuO4. In the present study, the one-dimensionality of the IC modulation, which is naturally explained by a stripe model, is clearly demonstrated with our “single-domain” crystal. The temperature dependence of the IC peak intensity suggests a substantial contribution from the Nd spins below ∼ 3 K. Consistent with this, the L dependence of the magnetic scattering is accurately accounted for by a model in which the contribution of the Nd spins is explicitly included.