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1.
Nano Lett ; 2020 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-32330042

RESUMO

The recent discovery of magnetic van der Waals (vdW) materials provides a platform to answer fundamental questions on the two-dimensional (2D) limit of magnetic phenomena and applications. An important question in magnetism is the ultimate limit of the antiferromagnetic layer thickness in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures to observe the exchange bias (EB) effect, of which origin has been subject to a long-standing debate. Here, we report that the EB effect is maintained down to the atomic bilayer of AFM in the FM (Fe3GeTe2)/AFM (CrPS4) vdW heterostructure, but it vanishes at the single-layer limit. Given that CrPS4 is of A-type AFM and, thus, the bilayer is the smallest unit to form an AFM, this result clearly demonstrates the 2D limit of EB; only one unit of AFM ordering is sufficient for a finite EB effect. Moreover, the semiconducting property of AFM CrPS4 allows us to electrically control the exchange bias, providing an energy-efficient knob for spintronic devices.

2.
Phys Rev Lett ; 124(2): 027601, 2020 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-32004043

RESUMO

The transition metal thiophosphates MPS_{3} (M=Mn, Fe, Ni) are a class of van der Waals stacked insulating antiferromagnets that can be exfoliated down to the ultrathin limit. MnPS_{3} is particularly interesting because its Néel ordered state breaks both spatial-inversion and time-reversal symmetries, allowing for a linear magnetoelectric phase that is rare among van der Waals materials. However, it is unknown whether this unique magnetic structure of bulk MnPS_{3} remains stable in the ultrathin limit. Using optical second harmonic generation rotational anisotropy, we show that long-range linear magnetoelectric type Néel order in MnPS_{3} persists down to at least 5.3 nm thickness. However an unusual mirror symmetry breaking develops in ultrathin samples on SiO_{2} substrates that is absent in bulk materials, which is likely related to substrate induced strain.

3.
Adv Mater ; : e1906578, 2020 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-32027057

RESUMO

Lattice distortion, spin interaction, and dimensional crossover in transition metal dichalcogenides (TMDs) have led to intriguing quantum phases such as charge density waves (CDWs) and 2D magnetism. However, the combined effect of many factors in TMDs, such as spin-orbit, electron-phonon, and electron-electron interactions, stabilizes a single quantum phase at a given temperature and pressure, which restricts original device operations with various quantum phases. Here, nontrivial polymorphic quantum states, CDW phases, are reported in vanadium ditelluride (VTe2 ) at room temperature, which is unique among various CDW systems; the doping concentration determines the formation of either of the two CDW phases in VTe2 at ambient conditions. The two CDW polymorphs show different antiferromagnetic spin orderings in which the vanadium atoms create two different stripe-patterned spin waves. First-principles calculations demonstrate that the magnetic ordering is critically coupled with the corresponding CDW in VTe2 , which suggests a rich phase diagram with polymorphic spin, charge, and lattice waves all coexisting in a solid for new conceptual quantum state-switching device applications.

4.
J Phys Condens Matter ; 32(12): 124003, 2020 Mar 20.
Artigo em Inglês | MEDLINE | ID: mdl-31770744

RESUMO

We present an overview of our recent work in tuning and controlling the structural, magnetic and electronic dimensionality of 2D van-der-Waals antiferromagnetic compounds (Transition-Metal)PS3. Low-dimensional magnetic systems such as these provide rich opportunities for studying new physics and the evolution of established behaviours with changing dimensionality. These materials can be exfoliated to monolayer thickness and easily stacked and combined into functional heterostructures. Alternatively, the application of hydrostatic pressure can be used to controllably close the van-der-Waals interplanar gap and tune the crystal structure and electron exchange paths towards a 3D nature. We collect and discuss trends and contrasts in our data from electrical transport, Raman scattering and synchrotron x-ray measurements, as well as insight from theoretical calculations and other results from the literature. We discuss structural transitions with pressure common to all materials measured, and link these to Mott insulator-transitions in these compounds at high pressures. Key new results include magnetotransport and resistivity data in the high-pressure metallic states, which show potentially interesting qualities for a new direction of future work focussed on low temperature transport and quantum critical physics.

5.
J Phys Condens Matter ; 32(3): 035601, 2020 Jan 16.
Artigo em Inglês | MEDLINE | ID: mdl-31561241

RESUMO

Two-dimensional layered transition-metal-dichalcogenide compound 1T-TaS2 shows the rare coexistence of charge density wave (CDW) and electron correlation driven Mott transition. In addition, atomic-cluster spins on the triangular lattice of the CDW state of 1T-TaS2 give rise to the possibility of the exotic spin-singlet state in which quantum fluctuations of spins are strong enough to prevent any long range magnetic ordering down to the temperature absolute zero (0 K). We present here the evidences of a glass-like random singlet magnetic state in 1T-TaS2 at low temperatures through a study of temperature and time dependence of magnetization. Comparing the experimental results with a representative canonical spin-glass system Au(1.8%Mn), we show that this glass-like state is distinctly different from the well established canonical spin-glass state.

6.
J Phys Condens Matter ; 31(50): 50LT01, 2019 Jul 11.
Artigo em Inglês | MEDLINE | ID: mdl-31295738

RESUMO

The widely-studied ferromagnetic van-der-Waals (vdW) metal Fe3GeTe2 has great promise for studies of quantum criticality in the 2D limit, but is limited by a relatively high Curie temperature in excess of 200 K. To help render the quantum critical point achievable in such a system within the reach of practically possible tuning methods, we have grown single crystals of a variant of (Fe,Co)3GeTe2 with useful physical properties for both this purpose and the wider study of low-dimensional magnetism and spin transport. (Fe,Co)3GeTe2 is found through x-ray diffraction and electron microscopy to have an equivalent crystal structure to Fe3GeTe2, with a random distribution of the cobalt dopant sites. It exhibits a sharp ferromagnetic transition at a value below 40 K, a stronger anisotropy and a coercive field ten times larger than that of Fe3GeTe2. The transport properties and specific heat show the electronic properties and strong correlations of Fe3GeTe2 to be near-unchanged in this doped material. We demonstrate that (Fe,Co)3GeTe2 can be cleanly exfoliated down to monolayer thickness. This unprecedented hard metallic vdW ferromagnet is a valuable new addition to the limited range of materials available for the study of 2D magnetism.

7.
Nat Commun ; 10(1): 345, 2019 01 21.
Artigo em Inglês | MEDLINE | ID: mdl-30664705

RESUMO

How a certain ground state of complex physical systems emerges, especially in two-dimensional materials, is a fundamental question in condensed-matter physics. A particularly interesting case is systems belonging to the class of XY Hamiltonian where the magnetic order parameter of conventional nature is unstable in two-dimensional materials leading to a Berezinskii-Kosterlitz-Thouless transition. Here, we report how the XXZ-type antiferromagnetic order of a magnetic van der Waals material, NiPS3, behaves upon reducing the thickness and ultimately becomes unstable in the monolayer limit. Our experimental data are consistent with the findings based on renormalization-group theory that at low temperatures a two-dimensional XXZ system behaves like a two-dimensional XY one, which cannot have a long-range order at finite temperatures. This work provides the experimental examination of the XY magnetism in the atomically thin limit and opens opportunities of exploiting these fundamental theorems of magnetism using magnetic van der Waals materials.

8.
ACS Nano ; 13(1): 552-559, 2019 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-30457832

RESUMO

Light-matter interactions in the van der Waals (vdWs) heterostructures exhibit many fascinating properties which can be harnessed to realize optoelectronic applications and probe fundamental physics. Moreover, the electron-phonon interaction in the vdWs heterostructures can have a profound impact on light-matter interaction properties because light excited electrons can strongly couple with phonons in heterostructures. Here, we report symmetry-controlled electron-phonon interactions in engineered two-dimensional (2D) material/silicon dioxide (SiO2) vdWs heterostructures. We observe two Raman modes arising from originally Raman-silent phonon modes in SiO2. The Raman modes have fixed peak positions regardless of the type of 2D materials in the heterostructures. Interestingly, such Raman emissions exhibit various symmetry properties in heterostructures with 2D materials of different crystalline structures, controlled by their intrinsic electronic band properties. In particular, we reveal chiral Raman emissions with reversed helicity in contrast to that of typical valley polarization in honeycomb 2D materials due to the phonon-assisted excitonic intervalley scattering process induced by electron-hole exchange interaction. The observation of the symmetry-controlled Raman scattering process not only provides a deep insight into the microscopic mechanisms of electron-phonon interactions in vdWs heterostructures but also may lead to the realization of valley-phononic devices.

9.
Nature ; 563(7729): 47-52, 2018 11.
Artigo em Inglês | MEDLINE | ID: mdl-30382199

RESUMO

The discovery of materials has often introduced new physical paradigms and enabled the development of novel devices. Two-dimensional magnetism, which is associated with strong intrinsic spin fluctuations, has long been the focus of fundamental questions in condensed matter physics regarding our understanding and control of new phases. Here we discuss magnetic van der Waals materials: two-dimensional atomic crystals that contain magnetic elements and thus exhibit intrinsic magnetic properties. These cleavable materials provide the ideal platform for exploring magnetism in the two-dimensional limit, where new physical phenomena are expected, and represent a substantial shift in our ability to control and investigate nanoscale phases. We present the theoretical background and motivation for investigating this class of crystals, describe the material landscape and the current experimental status of measurement techniques as well as devices, and discuss promising future directions for the study of magnetic van der Waals materials.

10.
Nano Lett ; 18(9): 5432-5438, 2018 09 12.
Artigo em Inglês | MEDLINE | ID: mdl-30063833

RESUMO

Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe2 on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe2. Renormalization-group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge-density wave-transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe2. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K that is driven by lattice distortions. The heterointerface plays an important role in driving this novel metal-insulator transition in the family of monolayer transition-metal dichalcogenides.

12.
Adv Mater ; 30(42): e1704777, 2018 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-29761925

RESUMO

The metal-insulator transition (MIT) in correlated materials is a novel phenomenon that accompanies a large change in resistivity, often many orders of magnitude. It is important in its own right but its switching behavior in resistivity can be useful for device applications. From the material physics point of view, the starting point of the research on the MIT should be to understand the microscopic mechanism. Here, an overview of recent efforts to unravel the microscopic mechanisms for various types of MITs in correlated materials is provided. Research has focused on transition metal oxides (TMOs), but transition metal chalcogenides have also been studied. Along the way, a new class of MIT materials is discovered, the so-called relativistic Mott insulators in 5d TMOs. Distortions in the MO6 (M = transition metal) octahedron are found to have a large and peculiar effect on the band structure in an orbital dependent way, possibly paving a way to the orbital selective Mott transition. In the final section, the character of the materials suitable for applications is summarized, followed by a brief discussion of some of the efforts to control MITs in correlated materials, including a dynamical approach using light.

13.
Phys Rev Lett ; 120(13): 136402, 2018 Mar 30.
Artigo em Inglês | MEDLINE | ID: mdl-29694193

RESUMO

Strong charge-spin coupling is found in a layered transition-metal trichalcogenide NiPS_{3}, a van der Waals antiferromagnet, from studies of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption, photoemission spectroscopy, and density functional calculations. NiPS_{3} displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission, and optical spectra support a self-doped ground state in NiPS_{3}. Our work demonstrates that layered transition-metal trichalcogenide magnets are useful candidates for the study of correlated-electron physics in two-dimensional magnetic materials.

14.
J Phys Condens Matter ; 30(23): 235802, 2018 Jun 13.
Artigo em Inglês | MEDLINE | ID: mdl-29697406

RESUMO

YFeO3 and LaFeO3 are members of the rare-earth orthoferrites family with Pbnm space group. Using inelastic neutron scattering, the low-energy spin excitations have been measured around the magnetic Brillouin zone center. Splitting of magnon branches and finite magnon gaps (∼2 meV) are observed for both compounds, where the Dzyaloshinsky-Moriya interactions account for most of this gap with some additional contribution from single-ion anisotropy. We also make comparisons with multiferroic BiFeO3 (R3c space group), in which similar behavior was observed. By taking into account all relevant local Dzyaloshinsky-Moriya interactions, our analysis allows for the precise determination of all experimentally observed parameters in the spin-Hamiltonian. We find that different properties of the Pbnm and R3c space group lead to the stabilization of a spin cycloid structure in the latter case but not in the former, which explains the difference in the levels of complexity of magnon band structures for the respective compounds.

15.
Sci Rep ; 8(1): 5092, 2018 Mar 23.
Artigo em Inglês | MEDLINE | ID: mdl-29572467

RESUMO

Most interesting phenomena of condensed matter physics originate from interactions among different degrees of freedom, making it a very intriguing yet challenging question how certain ground states emerge from only a limited number of atoms in assembly. This is especially the case for strongly correlated electron systems with overwhelming complexity. The Verwey transition of Fe3O4 is a classic example of this category, of which the origin is still elusive 80 years after the first report. Here we report, for the first time, that the Verwey transition of Fe3O4 nanoparticles exhibits size-dependent thermal hysteresis in magnetization, 57Fe NMR, and XRD measurements. The hysteresis width passes a maximum of 11 K when the size is 120 nm while dropping to only 1 K for the bulk sample. This behavior is very similar to that of magnetic coercivity and the critical sizes of the hysteresis and the magnetic single domain are identical. We interpret it as a manifestation of charge ordering and spin ordering correlation in a single domain. This work paves a new way of undertaking researches in the vibrant field of strongly correlated electron physics combined with nanoscience.

16.
Nano Lett ; 18(3): 1745-1750, 2018 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-29461844

RESUMO

57Fe nuclear magnetic resonance (NMR) of magnetite nanocrystals ranging in size from 7 nm to 7 µm is measured. The line width of the NMR spectra changes drastically around 120 K, showing microscopic evidence of the Verwey transition. In the region above the transition temperature, the line width of the spectrum increases and the spin-spin relaxation time decreases as the nanocrystal size decreases. The line-width broadening indicates the significant deformation of magnetic structure and reduction of charge order compared to bulk crystals, even when the structural distortion is unobservable. The reduction of the spin-spin relaxation time is attributed to the suppressed polaron hopping conductivity in ferromagnetic metals, which is a consequence of the enhanced electron-phonon coupling in the quantum-confinement regime. Our results show that the magnetic distortion occurs in the entire nanocrystal and does not comply with the simple model of the core-shell binary structure with a sharp boundary.

17.
J Phys Condens Matter ; 30(10): 105601, 2018 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-29380748

RESUMO

Hexagonal manganites are multiferroic materials with two highly-dissimilar phase transitions: a ferroelectric transition (from P63/mmc to P63cm) at a temperature higher than 1000 K and an antiferromagnetic transition at T N = 65-130 K. Despite its critical relevance to the intriguing ferroelectric domain physics, the details of the ferroelectric transition are not well known to date primarily because of the ultra-high transition temperature. Using high-temperature x-ray diffraction experiments, we show that the ferroelectric transition is a single transition of abrupt order and R-Op displacement is the primary order parameter. This structural transition is then simultaneously accompanied by MnO5 tilting and the subsequent development of electric polarization.

18.
J Phys Condens Matter ; 29(40): 405804, 2017 Oct 11.
Artigo em Inglês | MEDLINE | ID: mdl-28857048

RESUMO

We present measurements of resistivity, x-ray absorption (XAS) and emission (XES) spectroscopy together with ab initio band structure calculations for quasi two dimensional ruthenate Na2RuO3. Density function calculations (DFT) and XAS and XES spectra both show that Na2RuO3 is a semiconductor with an activation energy of ∼80 meV. Our DFT calculations reveal large magneto-elastic coupling in Na2RuO3 and predict that the ground state of Na2RuO3 should be antiferromagnetic zig-zag.

19.
Nat Commun ; 8: 15929, 2017 06 29.
Artigo em Inglês | MEDLINE | ID: mdl-28660878

RESUMO

The first known magnetic mineral, magnetite, has unusual properties, which have fascinated mankind for centuries; it undergoes the Verwey transition around 120 K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition, however, remains contentious. Here we use resonant inelastic X-ray scattering over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states. Comparison of the experimental results with crystal-field multiplet calculations shows that the spin-orbital dd excitons of the Fe2+ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe2+O6 octahedra. These low-energy excitations, which get weakened for temperatures above 350 K but persist at least up to 550 K, are distinct from optical excitations and are best explained as magnetic polarons.

20.
J Phys Condens Matter ; 29(13): 13LT01, 2017 Apr 05.
Artigo em Inglês | MEDLINE | ID: mdl-28140356

RESUMO

CuAl2O4 is a normal spinel oxide having quantum spin, S = 1/2 for Cu2+. It is a rather unique feature that the Cu2+ ions of CuAl2O4 sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4 shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards a higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu2+ is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f = |θ CW/T m| ~ 67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4 should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.

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