107 research outputs found
Confinement-Deconfinement Transition as an Indication of Spin-Liquid-Type Behavior in NaIrO
We use ultrafast optical spectroscopy to observe binding of charged
single-particle excitations (SE) in the magnetically frustrated Mott insulator
NaIrO. Above the antiferromagnetic ordering temperature () the
system response is due to both Hubbard excitons (HE) and their constituent
unpaired SE. The SE response becomes strongly suppressed immediately below
. We argue that this increase in binding energy is due to a unique
interplay between the frustrated Kitaev and the weak Heisenberg-type ordering
term in the Hamiltonian, mediating an effective interaction between the
spin-singlet SE. This interaction grows with distance causing the SE to become
trapped in the HE, similar to quark confinement inside hadrons. This binding of
charged particles, induced by magnetic ordering, is a result of a
confinement-deconfinement transition of spin excitations. This observation
provides evidence for spin liquid type behavior which is expected in
NaIrO.Comment: 5 pages, 3 figure
Recombination and propagation of quasiparticles in cuprate superconductors
Rapid developments in time-resolved optical spectroscopy have led to renewed interest in the nonequilibrium state of superconductors and other highly correlated electron materials. In these experiments, the nonequilibrium state is prepared by the absorption of short (less than 100 fs) laser pulses, typically in the near-infrared, that perturb the density and energy distribution of quasiparticles. The evolution of the nonequilibrium state is probed by time resolving the changes in the optical response functions of the medium that take place after photoexcitation. Ultimately, the goal of such experiments is to understand not only the nonequilibrium state, but to shed light on the still poorly understood equilibrium properties of these materials. We report nonequilibrium experiments that have revealed aspects of the cup rates that have been inaccessible by other techniques. Namely, the diffusion and recombination coefficients of quasiparticles have been measured in both YBa{sub 2}Cu{sub 3}O{sub 6.5} and Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+x} using time-resolved optical spectroscopy. Dependence of these measurements on doping, temperature and laser intensity is also obtained. To study the recombination of quasiparticles, we measure the change in reflectivity {Delta}R which is directly proportional to the nonequilibrium quasiparticle density created by the laser. From the intensity dependence, we estimate {beta}, the inelastic scattering coefficient and {gamma}{sub th} thermal equilibrium quasiparticle decay rate. We also present the dependence of recombination measurements on doping in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+x}. Going from underdoped to overdoped regime, the sign of {Delta}R changes from positive to negative right at the optimal doping. This is accompanied by a change in dynamics. The decay of {Delta}R stops being intensity dependent exactly at the optimal doping. We provide possible interpretations of these two observations. To study the propagation of quasiparticles, we interfered two laser pulses to introduce a spatially periodic density of quasiparticles. Probing the evolution of the initial density through space and time yielded the quasiparticle diffusion coefficient, and both inelastic and elastic scattering rates. Measured diffusion coefficient suggests that the quasiparticles induced by the laser occupy primarily states near the antinodal regions of the Brillouin zone
The origin of exciton mass in a frustrated Mott insulator NaIrO
We use a three-pulse ultrafast optical spectroscopy to study the relaxation
processes in a frustrated Mott insulator NaIrO. By being able to
independently produce the out-of-equilibrium bound states (excitons) of
doublons and holons with the first pulse and suppress the underlying
antiferromagnetic order with the second one, we were able to elucidate the
relaxation mechanism of quasiparticles in this system. By observing the
difference in the exciton dynamics in the magnetically ordered and disordered
phases we found that the mass of this quasiparticle is mostly determined by its
interaction with the surrounding spins
Fluctuating charge density waves in a cuprate superconductor
Cuprate materials hosting high-temperature superconductivity (HTS) also
exhibit various forms of charge and/or spin ordering whose significance is not
fully understood. To date, static charge-density waves (CDWs) have been
detected by diffraction probes only at special doping or in an applied external
field. However, dynamic CDWs may also be present more broadly and their
detection, characterization and relationship with HTS remain open problems.
Here, we present a new method, based on ultrafast spectroscopy, to detect the
presence and measure the lifetimes of CDW fluctuations in cuprates. In an
underdoped La1.9Sr0.1CuO4 film (Tc = 26 K), we observe collective excitations
of CDW that persist up to 100 K. This dynamic CDW fluctuates with a
characteristic lifetime of 2 ps at T = 5 K which decreases to 0.5 ps at T = 100
K. In contrast, in an optimally doped La1.84Sr0.16CuO4 film (Tc = 38.5 K), we
detect no signatures of fluctuating CDWs at any temperature, favoring the
competition scenario. This work forges a path for studying fluctuating order
parameters in various superconductors and other materials.Comment: 16 pages, 4 figures, accepted to Nature Material
Observation of intervalley biexcitonic optical Stark effect in monolayer WS2
Coherent optical dressing of quantum materials offers technological
advantages to control their electronic properties, such as the electronic
valley degree of freedom in monolayer transition metal dichalcogenides (TMDs).
Here, we observe a new type of optical Stark effect in monolayer WS2, one that
is mediated by intervalley biexcitons under the blue-detuned driving with
circularly polarized light. We found that such helical optical driving not only
induces an exciton energy downshift at the excitation valley, but also causes
an anomalous energy upshift at the opposite valley, which is normally forbidden
by the exciton selection rules but now made accessible through the intervalley
biexcitons. These findings reveal the critical, but hitherto neglected, role of
biexcitons to couple the two seemingly independent valleys, and to enhance the
optical control in valleytronics
Confinement-Deconfinement Transition as an Indication of Spin-Liquid-Type Behavior in Na\u3csub\u3e2\u3c/sub\u3eIrO\u3csub\u3e3\u3c/sub\u3e
We use ultrafast optical spectroscopy to observe binding of charged single-particle excitations (SE) in the magnetically frustrated Mott insulator Na2IrO3. Above the antiferromagnetic ordering temperature (TN) the system response is due to both Hubbard excitons (HE) and their constituent unpaired SE. The SE response becomes strongly suppressed immediately below TN. We argue that this increase in binding energy is due to a unique interplay between the frustrated Kitaev and the weak Heisenberg-type ordering term in the Hamiltonian, mediating an effective interaction between the spin-singlet SE. This interaction grows with distance causing the SE to become trapped in the HE, similar to quark confinement inside hadrons. This binding of charged particles, induced by magnetic ordering, is a result of a confinement-deconfinement transition of spin excitations. This observation provides evidence for spin liquid type behavior which is expected in Na2IrO3
Absence of Magnetic Fluctuations in the Ferromagnetic/Topological Heterostructure EuS/BiSe
Heterostructures of topological insulators and ferromagnets offer new
opportunities in spintronics and a route to novel anomalous Hall states. In one
such structure, EuS/BiSe a dramatic enhancement of the Curie
temperature was recently observed. We performed Raman spectroscopy on a similar
set of thin films to investigate the magnetic and lattice excitations.
Interfacial strain was monitored through its effects on the BiSe
phonon modes while the magnetic system was probed through the EuS Raman mode.
Despite its appearance in bare EuS, the heterostructures lack the corresponding
EuS Raman signal. Through numerical calculations we rule out the possibility of
Fabry-Perot interference suppressing the mode. We attribute the absence of a
magnetic signal in EuS to a large charge transfer with the BiSe.
This could provide an additional pathway for manipulating the magnetic,
optical, or electronic response of topological heterostructures.Comment: 6 pages, 3 figure
Valley-selective optical Stark effect in monolayer WS2
Breaking space-time symmetries in two-dimensional crystals (2D) can
dramatically influence their macroscopic electronic properties. Monolayer
transition-metal dichalcogenides (TMDs) are prime examples where the
intrinsically broken crystal inversion symmetry permits the generation of
valley-selective electron populations, even though the two valleys are
energetically degenerate, locked by time-reversal symmetry. Lifting the valley
degeneracy in these materials is of great interest because it would allow for
valley-specific band engineering and offer additional control in valleytronic
applications. While applying a magnetic field should in principle accomplish
this task, experiments to date have observed no valley-selective energy level
shifts in fields accessible in the laboratory. Here we show the first direct
evidence of lifted valley degeneracy in the monolayer TMD WS2. By applying
intense circularly polarized light, which breaks time-reversal symmetry, we
demonstrate that the exciton level in each valley can be selectively tuned by
as much as 18 meV via the optical Stark effect. These results offer a novel way
to control valley degree of freedom, and may provide a means to realize new
valley-selective Floquet topological phases in 2D TMDs
Amplitude dynamics of charge density wave in LaTe: theoretical description of pump-probe experiments
We formulate a dynamical model to describe a photo-induced charge density
wave (CDW) quench transition and apply it to recent multi-probe experiments on
LaTe [A. Zong et al., Nat. Phys. 15, 27 (2019)]. Our approach relies on
coupled time-dependent Ginzburg-Landau equations tracking two order parameters
that represent the modulations of the electronic density and the ionic
positions. We aim at describing the amplitude of the order parameters under the
assumption that they are homogeneous in space. This description is supplemented
by a three-temperature model, which treats separately the electronic
temperature, temperature of the lattice phonons with stronger couplings to the
electronic subsystem, and temperature of all other phonons. The broad scope of
available data for LaTe and similar materials as well as the synergy
between different time-resolved spectroscopies allow us to extract model
parameters. The resulting calculations are in good agreement with ultra-fast
electron diffraction experiments, reproducing qualitative and quantitative
features of the CDW amplitude evolution during the initial few picoseconds
after photoexcitation.Comment: 21 pages, 14 figures; this version is almost identical to the
published version; comparing to the earlier arXiv submission, current version
contains a new figure (Fig.10), and a broader discussion of theoretical
results and approximation
Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of SrTiO
The combination of ferromagnets with topological superconductors or
insulators allows for new phases of matter that support excitations such as
chiral edge modes and Majorana fermions. EuS, a wide-band-gap ferromagnetic
insulator with a Curie temperature around 16 K, and SrTiO (STO), an
important substrate for engineering heterostructures, may support these phases.
We present scanning superconducting quantum interference device (SQUID)
measurements of EuS grown epitaxially on STO that reveal micron-scale
variations in ferromagnetism and paramagnetism. These variations are oriented
along the STO crystal axes and only change their configuration upon thermal
cycling above the STO cubic-to-tetragonal structural transition temperature at
105 K, indicating that the observed magnetic features are due to coupling
between EuS and the STO tetragonal structure. We speculate that the STO
tetragonal distortions may strain the EuS, altering the magnetic anisotropy on
a micron-scale. This result demonstrates that local variation in the induced
magnetic order from EuS grown on STO needs to be considered when engineering
new phases of matter that require spatially homogeneous exchange
- …