14 research outputs found
Cavity-Enhanced Linear Dichroism in a van der Waals Antiferromagnet
Optical birefringence is a fundamental optical property of crystals widely
used for filtering and beam splitting of photons. Birefringent crystals
concurrently possess the property of linear dichroism (LD) that allows
asymmetric propagation or attenuation of light with two different
polarizations. This property of LD has been widely studied from small molecules
to polymers and crystals but has rarely been engineered per will. Here, we use
the newly discovered spin-charge coupling in van der Waals antiferromagnetic
(AFM) insulator FePS3 to induce large in-plane optical anisotropy and
consequently LD. We report that the LD in this AFM insulator is tunable both
spectrally and magnitude-wise as a function of cavity coupling. We demonstrate
near-unity LD in the visible-near infrared range in cavity-coupled FePS3
crystals and derive its dispersion as a function of cavity length and FePS3
thickness. Our results hold wide implications for use of cavity tuned LD as a
diagnostic probe for strongly correlated quantum materials as well as opens new
opportunities for miniaturized, on-chip beam-splitters and tunable filters.Comment: 14 pages, 5 figure
Observation of terahertz second harmonic generation from Dirac surface states in the topological insulator BiSe
We report the observation of second harmonic generation with high conversion
efficiency in the terahertz regime from thin films of the
topological insulator BiSe that exhibit the linear photogalvanic
effect, measured via time-domain terahertz spectroscopy and terahertz emission,
respectively. As neither phenomena is observable from topologically trivial
In-doped BiSe, and since no enhancement is observed when subject to
band bending, the efficient thickness-independent nonliear responses are
attributable to the Dirac fermions of topological surface states of
BiSe. This observation of intrinsic terahertz second harmonic
generation in an equilibrium system unlocks the full suite of both even and odd
harmonic orders in the terahertz regime and opens new pathways to probing
quantum geometry via intraband nonlinear processes.Comment: 8 pages, 3 figure
Linear and nonlinear optical responses in the chiral multifold semimetal RhSi
Chiral topological semimetals are materials that break both inversion and
mirror symmetries. They host interesting phenomena such as the quantized
circular photogalvanic effect (CPGE) and the chiral magnetic effect. In this
work, we report a comprehensive theoretical and experimental analysis of the
linear and non-linear optical responses of the chiral topological semimetal
RhSi, which is known to host multifold fermions. We show that the
characteristic features of the optical conductivity, which display two distinct
quasi-linear regimes above and below 0.4 eV, can be linked to excitations of
different kinds of multifold fermions. The characteristic features of the CPGE,
which displays a sign change at 0.4 eV and a large non-quantized response peak
of around 160 at 0.7 eV, are explained by assuming that
the chemical potential crosses a flat hole band at the Brillouin zone center.
Our theory predicts that, in order to observe a quantized CPGE in RhSi, it is
necessary to increase the chemical potential as well as the quasiparticle
lifetime. More broadly our methodology, especially the development of the
broadband terahertz emission spectroscopy, could be widely applied to study
photo-galvanic effects in noncentrosymmetric materials and in topological
insulators in a contact-less way and accelerate the technological development
of efficient infrared detectors based on topological semimetals.Comment: Accepted in npj Quantum Materials; Abstract update
Universal three-state nematicity and magneto-optical Kerr effect in the charge density waves in AVSb (A=Cs, Rb, K)
The kagome lattice provides a fascinating playground to study geometrical
frustration, topology and strong correlations. The newly discovered kagome
metals AVSb (A=K, Rb, Cs) exhibit various interesting phenomena
including topological band structure and superconductivity. Nevertheless, the
nature of the symmetry breaking in the CDW phase is not yet clear, despite the
fact that it is crucial to understand whether the superconductivity is
unconventional. In this work, we perform scanning birefringence microscopy and
find that six-fold rotation symmetry is broken at the onset of the CDW
transition temperature in all three compounds. Spatial imaging and angle
dependence of the birefringence show a universal three nematic domains that are
120 to each other. We propose staggered CDW orders with a relative
phase shift between layers as a possibility to explain the three-state
nematicity in AVSb. We also perform magneto-optical Kerr effect and
circular dichroism measurements on all three compounds, and the onset of the
both signals is at the CDW transition temperature, indicating broken
time-reversal symmetry and the existence of the long-sought loop currents in
the CDW phase. Our work strongly constrains the nature of the CDWs and sheds
light on possible unconventional superconductivity in AVSb.Comment: 7 pages, 4 figures, submitted versio
Giant topological longitudinal circular photo-galvanic effect in the chiral multifold semimetal CoSi
The absence of mirror symmetry, or chirality, is behind striking natural
phenomena found in systems as diverse as DNA and crystalline solids. A
remarkable example occurs when chiral semimetals with topologically protected
band degeneracies are illuminated with circularly polarized light. Under the
right conditions, the part of the generated photocurrent that switches sign
upon reversal of the light's polarization, known as the circular photogalvanic
effect, is predicted to depend only on fundamental constants. The conditions to
observe quantization are non-universal, and depend on material parameters and
the incident frequency. In this work, we perform terahertz emission
spectroscopy with tunable photon energy from 0.2 eV - 1.1 eV in the chiral
topological semimetal CoSi. We identify a large longitudinal photocurrent
peaked at 0.4 eV reaching 550 , which is much larger than
the photocurrent in any chiral crystal reported in the literature. Using
first-principles calculations we establish that the peak originates from
topological band crossings, reaching 3.30.3 in units of the quantization
constant. Our calculations indicate that the quantized CPGE is within reach in
CoSi upon doping and increase of the hot-carrier lifetime. The large
photo-conductivity suggests that topological semimetals could potentially be
used as novel mid-infrared detectors.Comment: Fig.4 color update
Signatures of Z Vestigial Potts-nematic order in van der Waals antiferromagnets
Layered van der Waals magnets have attracted much recent attention as a
promising and versatile platform for exploring intrinsic two-dimensional
magnetism. Within this broader class, the transition metal phosphorous
trichalcogenides P stand out as particularly interesting, as they
provide a realization of honeycomb lattice magnetism and are known to display a
variety of magnetic ordering phenomena as well as superconductivity under
pressure. One example, found in a number of different materials, is
commensurate single- zigzag antiferromagnetic order, which spontaneously
breaks the spatial threefold rotation symmetry of the honeycomb
lattice. The breaking of multiple distinct symmetries in the magnetic phase
suggests the possibility of a sequence of distinct transitions as a function of
temperature, and a resulting intermediate -nematic phase which
exists as a paramagnetic vestige of zigzag magnetic order -- a scenario known
as vestigial ordering. Here, we report the observation of key signatures of
vestigial Potts-nematic order in rhombohedral FePSe. By performing linear
dichroism imaging measurements -- an ideal probe of rotational symmetry
breaking -- we find that the symmetry is already broken above the N\'eel
temperature. We show that these observations are explained by a general
Ginzburg-Landau model of vestigial nematic order driven by magnetic
fluctuations and coupled to residual strain. An analysis of the domain
structure as temperature is lowered and a comparison with zigzag-ordered
monoclinic FePS reveals a broader applicability of the Ginzburg-Landau
model in the presence of external strain, and firmly establishes the P
magnets as a new experimental venue for studying the interplay between
Potts-nematicity, magnetism and superconductivity.Comment: 6 pages, 4 figures + supplementary materia
Observation of giant surface second harmonic generation coupled to nematic orders in the van der Waals antiferromagnet FePS
Second harmonic generation has been applied to study lattice, electronic and
magnetic proprieties in atomically thin materials. However, inversion symmetry
breaking is usually required for the materials to generate a large signal. In
this work, we report a giant second-harmonic generation that arises below the
N\'eel temperature in few-layer centrosymmetric FePS. Layer-dependent study
indicates the detected signal is from the second-order nonlinearity of the
surface. The magnetism-induced surface second-harmonic response is two orders
of magnitude larger than those reported in other magnetic systems, with the
surface nonlinear susceptibility reaching 0.08--0.13 nm/V in 2 L--5 L
samples. By combing linear dichroism and second harmonic generation
experiments, we further confirm the giant second-harmonic generation is coupled
to nematic orders formed by the three possible Zigzag antiferromagnetic
domains. Our study shows that the surface second-harmonic generation is also a
sensitive tool to study antiferromagnetic states in centrosymmetric atomically
thin materials.Comment: to appear in Nano Letter
Atomic ruthenium-riveted metal-organic framework with tunable d-band modulates oxygen redox for lithium-oxygen batteries
Non-aqueous Li-O2 batteries have aroused considerable attention because of their ultrahigh theoretical energy density, but they are severely hindered by slow cathode reaction kinetics and large overvoltages, which are closely associated with the discharge product of Li2O2. Herein, hexagonal conductive metal-organic framework nanowire arrays of nickel-hexaiminotriphenylene (Ni-HTP) with quadrilateral Ni-N4 units are synthesized to incorporate Ru atoms into its skeleton for NiRu-HTP. The atomically dispersed Ru-N4 sites manifest strong adsorption for the LiO2 intermediate owing to its tunable d-band center, leading to its high local concentration around NiRu-HTP. This favors the formation of film-like Li2O2 on NiRu-HTP with promoted electron transfer and ion diffusion across the cathode-electrolyte interface, facilitating its reversible decomposition during charge. These allow the Li-O2 battery with NiRu-HTP to deliver a remarkably reduced charge/discharge polarization of 0.76 V and excellent cyclability. This work will enrich the design philosophy of electrocatalysts for regulation of kinetic behaviors of oxygen redox.This work was supported by National Natural Science Foundation of China (52171215), the Tianjin Natural Science Foundation (19JCJQJC62400), and Haihe Laboratory of Sustainable Chemical Transformations