10 research outputs found
Magnonic superradiant phase transition
磁石の中で自然と現れる「止まった波」 --超放射相転移が起こる磁石を発見--. 京都大学プレスリリース. 2022-01-12.In the superradiant phase transition (SRPT), coherent light and matter fields are expected to appear spontaneously in a coupled light–matter system in thermal equilibrium. However, such an equilibrium SRPT is forbidden in the case of charge-based light–matter coupling, known as no-go theorems. Here, we show that the low-temperature phase transition of ErFeO₃ at a critical temperature of approximately 4 K is an equilibrium SRPT achieved through coupling between Fe³⁺ magnons and Er³⁺ spins. By verifying the efficacy of our spin model using realistic parameters evaluated via terahertz magnetospectroscopy and magnetization experiments, we demonstrate that the cooperative, ultrastrong magnon–spin coupling causes the phase transition. In contrast to prior studies on laser-driven non-equilibrium SRPTs in atomic systems, the magnonic SRPT in ErFeO₃ occurs in thermal equilibrium in accordance with the originally envisioned SRPT, thereby yielding a unique ground state of a hybrid system in the ultrastrong coupling regime
Time-Domain Terahertz Spectroscopy in High Magnetic Fields
There are a variety of elementary and collective terahertz-frequency
excitations in condensed matter whose magnetic field dependence contains
significant insight into the states and dynamics of the electrons involved.
Often, determining the frequency, temperature, and magnetic field dependence of
the optical conductivity tensor, especially in high magnetic fields, can
clarify the microscopic physics behind complex many-body behaviors of solids.
While there are advanced terahertz spectroscopy techniques as well as high
magnetic field generation techniques available, combination of the two has only
been realized relatively recently. Here, we review the current state of
terahertz time-domain spectroscopy experiments in high magnetic fields. We
start with an overview of time-domain terahertz detection schemes with a
special focus on how they have been incorporated into optically accessible
high-field magnets. Advantages and disadvantages of different types of magnets
in performing terahertz time-domain spectroscopy experiments are also
discussed. Finally, we highlight some of the new fascinating physical phenomena
that have been revealed by terahertz time-domain spectroscopy in high magnetic
fields
Perfect Intrinsic Squeezing at the Superradiant Phase Transition Critical Point
Some of the most exotic properties of the quantum vacuum are predicted in ultrastrongly coupled photon–atom systems; one such property is quantum squeezing leading to suppressed quantum fluctuations of photons and atoms. This squeezing is unique because (1) it is realized in the ground state of the system and does not require external driving, and (2) the squeezing can be perfect in the sense that quantum fluctuations of certain observables are completely suppressed. Specifically, we investigate the ground state of the Dicke model, which describes atoms collectively coupled to a single photonic mode, and we found that the photon–atom fluctuation vanishes at the onset of the superradiant phase transition in the thermodynamic limit of an infinite number of atoms. Moreover, when a finite number of atoms is considered, the variance of the fluctuation around the critical point asymptotically converges to zero, as the number of atoms is increased. In contrast to the squeezed states of flying photons obtained using standard generation protocols with external driving, the squeezing obtained in the ground state of the ultrastrongly coupled photon–atom systems is resilient against unpredictable noise
Observation of Colossal Terahertz Magnetoresistance and Magnetocapacitance in a Perovskite Manganite
We have studied the terahertz response of a bulk single crystal of
LaSrMnO at around its Curie temperature, observing
large changes in the real and imaginary parts of the optical conductivity as a
function of magnetic field. The terahertz resistance and capacitance extracted
from the optical conductivity rapidly increased with increasing magnetic field
and did not show any sign of saturation up to 6 T, reaching 60% and 15%,
respectively, at 180 K. The observed terahertz colossal magnetoresistance and
magnetocapacitance effects can be qualitatively explained by using a
two-component model that assumes the coexistence of two phases with vastly
different conductivities. These results demonstrate the potential use of
perovskite manganites for developing efficient terahertz devices based on
magnetic modulations of the amplitude and phase of terahertz waves.Comment: 7 pages, 6 figure
Terahertz Faraday and Kerr rotation spectroscopy of BiSb films in high magnetic fields up to 30 Tesla
We report results of terahertz Faraday and Kerr rotation spectroscopy
measurements on thin films of , an alloy system
that exhibits a semimetal-to-topological-insulator transition as the Sb
composition increases. By using a single-shot time-domain terahertz
spectroscopy setup combined with a table-top pulsed mini-coil magnet, we
conducted measurements in magnetic fields up to 30~T, observing distinctly
different behaviors between semimetallic () and topological insulator
() samples. Faraday and Kerr rotation spectra for the semimetallic
films showed a pronounced dip that blue-shifted with the magnetic field,
whereas spectra for the topological insulator films were positive and
featureless, increasing in amplitude with increasing magnetic field and
eventually saturating at high fields (20~T). Ellipticity spectra for the
semimetallic films showed resonances, whereas the topological insulator films
showed no detectable ellipticity. To explain these observations, we developed a
theoretical model based on realistic band parameters and the Kubo formula for
calculating the optical conductivity of Landau-quantized charge carriers. Our
calculations quantitatively reproduced all experimental features, establishing
that the Faraday and Kerr signals in the semimetallic films predominantly arise
from bulk hole cyclotron resonances while the signals in the topological
insulator films represent combined effects of surface carriers originating from
multiple electron and hole pockets. These results demonstrate that the use of
high magnetic fields in terahertz magnetopolarimetry, combined with detailed
electronic structure and conductivity calculations, allows us to unambiguously
identify and quantitatively determine unique contributions from different
species of carriers of topological and nontopological nature in
BiSb.Comment: 17 pages, 22 figure
Supplementary document for Observation of Colossal Terahertz Magnetoresistance and Magnetocapacitance in a Perovskite Manganite - 6434884.pdf
Supplementary Material
Ultrastrong magnon-magnon coupling dominated by antiresonant interactions
Makihara T, Hayashida K, Noe Ii GT, et al. Ultrastrong magnon-magnon coupling dominated by antiresonant interactions. Nature communications. 2021;12(1): 3115.Exotic quantum vacuum phenomena are predicted in cavity quantum electrodynamics systems with ultrastrong light-matter interactions. Their ground states are predicted to be vacuum squeezed states with suppressed quantum fluctuations owing to antiresonant terms in the Hamiltonian. However, such predictions have not been realized because antiresonant interactions are typically negligible compared to resonant interactions in light-matter systems. Here we report an unusual, ultrastrongly coupled matter-matter system of magnons that is analytically described by a unique Hamiltonian in which the relative importance of resonant and antiresonant interactions can be easily tuned and the latter can be made vastly dominant. We found a regime where vacuum Bloch-Siegert shifts, the hallmark of antiresonant interactions, greatly exceed analogous frequency shifts from resonant interactions. Further, we theoretically explored the system's ground state and calculated up to 5.9 dB of quantum fluctuation suppression. These observations demonstrate that magnonic systems provide an ideal platform for exploring exotic quantum vacuum phenomena predicted in ultrastrongly coupled light-matter systems