15 research outputs found
Tunable interband and intraband plasmons in twisted double bilayer graphene
Flat bands in twisted moire superlattices support a variety of topological
and strongly correlated phenomena along with easily tunable electrical and
optical properties. Here, we demonstrate the existence of tunable, long-lived,
and flat intraband and interband terahertz plasmons in twisted double bilayer
graphene. We show that the interband plasmons originate from the presence of a
Van Hove singularity in the joint density of states and a finite Berry
connection between the pair of bands involved. We find that the gapped
interband plasmon mode has a universal dispersion, and the plasmon gap is
specified by the location of the Van Hove singularity in the joint density of
states. Metallic moire systems support an additional intraband plasmon mode
which becomes flat in the large momentum limit because of the influence of the
interband correlations. We demonstrate that the undamped and flat plasmon modes
in moire systems are highly tunable and can be controlled by varying the
vertical electric field and electron doping, and they persist over a wide range
of twist angles.Comment: 4 figures, 9 page
Intrinsic nonreciprocal bulk plasmons in noncentrosymmetric magnetic systems
Nonreciprocal plasmonics enables one-way light propagation at the nanoscale
and it is an essential building block for photonics applications. Here, we
explore intrinsic nonreciprocity in bulk plasmon propagation based on
underlying symmetries. We demonstrate that the interband, as well as the
intraband bulk plasmon modes, follow asymmetric dispersion depending on the
sign of the wavevector for systems with broken inversion and time-reversal
symmetry. We show that the nonreciprocity in the interband plasmon dispersion
is dictated by the quantum metric connection, which is a band geometric
quantity. The intrinsic nonreciprocity in bulk intraband plasmon dispersion is
dictated by the quantum metric dipole and a higher-order `Drude' weight-like
term. We corroborate our findings via explicit numerical calculations for the
two-dimensional Qi-Wu-Zhang model and demonstrate the existence of intrinsic
nonreciprocal intraband and interband plasmon modes in moire systems such as
twisted bilayer graphene.Comment: 14 pages, 6 figure
Room Temperature Ferroelectricity, Ferromagnetism, and Anomalous Hall Effect in Half-metallic Monolayer CrTe
Two-dimensional materials hosting ferroelectricity and ferromagnetism are
crucial for low-power and high-speed information processing technologies.
However, intrinsic 2D multiferroics in the monolayer limit are rare. Here, we
demonstrate that monolayer CrTe, obtained by cleaving the [002] surface, is
dynamically stable multiferroic at temperatures beyond room temperature. We
show that it orders ferromagnetically with significant in-plane
magnetocrystalline anisotropy, and it is a half-metal featuring a large
half-metal gap. Remarkably, the broken inversion symmetry and buckled geometry
of monolayer CrTe make it a ferroelectric with a large spontaneous out-of-plane
polarization and significant magnetoelectric coupling. In addition, we
demonstrate polarization or electric field-induced tunability of the anomalous
Hall effect, accompanied by substantial bandstructure modulation. Our findings
establish monolayer CrTe as a room-temperature multiferroic with great
potential for applications in spintronics and ferroelectric devices.Comment: 9 double column pages, and 5 figures; A significantly updated version
predicting monolayer CrTe to be an intrinsic multiferroic at room temperatur
Observation of time-reversal symmetric Hall effect in graphene-WSe2 heterostructures at room temperature
In this letter, we provide experimental evidence of the time-reversal
symmetric Hall effect in a mesoscopic system, namely high-mobility
graphene/WSe heterostructures. This linear, dissipative Hall effect, whose
sign depends on the sign of the charge carriers, persists up to room
temperature. The magnitude and the sign of the Hall signal can be tuned using
an external perpendicular electric field. Our joint experimental and
theoretical study establishes that the strain induced by lattice mismatch, or
angle inhomogeneity, produces anisotropic bands in graphene while
simultaneously breaking the inversion symmetry. The band anisotropy and reduced
spatial symmetry lead to the appearance of a time-reversal symmetric Hall
effect. Our study establishes graphene-transition metal dichalcogenide-based
heterostructures as an excellent platform for studying the effects of broken
symmetry on the physical properties of band-engineered two-dimensional systems
Origin of magnetic moments and presence of a resonating valence bond state in BaYIrO
While it was speculated that 5 systems would possess non-magnetic
~=~0 ground state due to strong Spin-Orbit Coupling (SOC), all such systems
have invariably shown presence of magnetic moments so far. A puzzling case is
that of BaYIrO, which in spite of having a perfectly cubic structure
with largely separated Ir () ions, has consistently shown presence
of weak magnetic moments. Moreover, we clearly show from Muon Spin Relaxation
(SR) measurements that a change in the magnetic environment of the
implanted muons in BaYIrO occurs as temperature is lowered below 10~K.
This observation becomes counterintuitive, as the estimated value of SOC
obtained by fitting the RIXS spectrum of BaYIrO with an atomic
model is found to be as high as 0.39~eV, meaning that the system within this
model is neither expected to possess moments nor exhibit temperature dependent
magnetic response. Therefore we argue that the atomic coupling
description is not sufficient to explain the ground state of such systems,
where despite having strong SOC, presence of hopping triggers delocalisation of
holes, resulting in spontaneous generation of magnetic moments. Our theoretical
calculations further indicate that these moments favour formation of
spin-orbital singlets in the case of BaYIrO, which is manifested in
SR experiments measured down to 60~mK.Comment: 20 Pages, 7 Figure
Unconventional magnetism in the 4d based () honeycomb system AgLiRuO
We have investigated the thermodynamic and local magnetic properties of the
Mott insulating system AgLiRuO containing Ru
(4) for novel magnetism. The material crystallizes in a monoclinic
structure with RuO octahedra forming an edge-shared
two-dimensional honeycomb lattice with limited stacking order along the
-direction. The large negative Curie-Weiss temperature ( = -57
K) suggests antiferromagnetic interactions among Ru ions though magnetic
susceptibility and heat capacity show no indication of magnetic long-range
order down to 1.8 K and 0.4 K, respectively. Li nuclear magnetic
resonance (NMR) shift follows the bulk susceptibility between 120-300 K and
levels off below 120 K. Together with a power-law behavior in the temperature
dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic
spin correlations with gapless excitations. Electronic structure calculations
suggest an description of the Ru-moments and the possible importance of
further neighbour interactions as also bi-quadratic and ring-exchange terms in
determining the magnetic properties. Analysis of our SR data indicates
spin freezing below 5 K but the spins remain on the borderline between static
and dynamic magnetism even at 20 mK.Comment: 10 pages, 11 figures. accepted in Phys. Rev.
Gapless quantum spin liquid in the triangular system SrCuSbO
We report gapless quantum spin liquid behavior in the layered triangular
SrCuSbO (SCSO) system. X-ray diffraction shows superlattice
reflections associated with atomic site ordering into triangular Cu planes
well-separated by Sb planes. Muon spin relaxation (SR) measurements show
that the moments at the magnetically active Cu sites remain
dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale
evidenced by a large Curie-Weiss temperature
-143 K as extracted from the bulk susceptibility. Specific heat measurements
also show no sign of long-range order down to 0.35 K. The magnetic specific
heat () below 5 K reveals a
+ behavior. The
significant contribution to the magnetic specific heat invites a
phenomenology in terms of the so-called Dirac spinon excitations with a linear
dispersion. From the low- specific heat data, we estimate the dominant
exchange scale to be 36 K using a Dirac spin liquid ansatz which is not
far from the values inferred from microscopic density functional theory
calculations ( 45 K) as well as high-temperature susceptibility analysis
( 70 K). The linear specific heat coefficient is about 18 mJ/mol-K
which is somewhat larger than for typical Fermi liquids.Comment: 16 pages, 21 figures, including supplementary material. A Dirac spin liquid scenario has been put forward to explain the
field-dependent specific heat data. Comments are welcom
Direct observation of altermagnetic band splitting in CrSb thin films
Altermagnetism represents an emergent collinear magnetic phase with
compensated order and an unconventional alternating even-parity wave spin order
in the non-relativistic band structure. We investigate directly this
unconventional band splitting near the Fermi energy through spinintegrated soft
X-ray angular resolved photoemission spectroscopy. The experimentally obtained
angle-dependent photoemission intensity, acquired from epitaxial thin films of
the predicted altermagnet CrSb, demonstrates robust agreement with the
corresponding band structure calculations. In particular, we observe the
distinctive splitting of an electronic band on a low-symmetry path in the
Brilliouin zone that connects two points featuring symmetry-induced degeneracy.
The measured large magnitude of the spin splitting of approximately 0.6 eV and
the position of the band just below the Fermi energy underscores the
signifcance of altermagnets for spintronics based on robust broken time
reversal symmetry responses arising from exchange energy scales, akin to
ferromagnets, while remaining insensitive to external magnetic fields and
possessing THz dynamics, akin to antiferromagnets.Comment: 10 pages, 7 figures (including supplementary information