Ferromagnetic/antiferromagnetic
materials are of crucial importance
in information storage and spintronics devices. Herein we present
a comprehensive study of 2D Heisenberg-like antiferromagnetic material
MnPS3 by optical contrast and Raman spectroscopy. We propose
a criterion of 0.1 × (N – 1) < (ΔR/R)max < 0.1 × N (N ≤ 7) to quickly identify the
layer number N by using maximum optical contrast
(ΔR/R)max of few-layer
MnPS3 on a SiO2/Si substrate (90 nm thick SiO2). The Raman modes are also identified by polarization Raman
spectroscopy. Furthermore, by temperature-dependent Raman measurements,
we obtain three phase transition temperatures of MnPS3.
The transition temperature at around 80 K corresponds to the transition
from the antiferromagnetic to paramagnetic phase; the one at around
120 K is related to its second magnetic phase transition temperature
due to two-dimensional spin critical fluctuations; the one at around
55 K is associated with unbinding of spin vortices. Our studies provide
more evidence to advance knowledge of the magnetic critical dynamics
of 2D ferromagnetic/antiferromagnetic systems.
Phonon-assisted
anti-Stokes photoluminescence (ASPL) up-conversion
lies at the heart of optical refrigeration in solids. The thermal
energy contained in the lattice vibrations is taken away by the emitted
anti-Stokes photons’ ASPL process, resulting in laser cooling
of solids. To date, net laser cooling of solids is limited in rare-earth
(RE)-doped crystals, glasses, and direct band gap semiconductors.
Searching more solid materials with efficient phonon-assisted photoluminescence
up-conversion is important to enrich optical refrigeration research.
Here, we demonstrate the phonon-assisted PL up-conversion process
from the silicon vacancy (SiV) center in diamond for the first time
by studying ASPL spectra for the dependence of temperature, laser
power, and excitation energy. Although net cooling has not been observed,
our results show that net laser cooling might be eventually achieved
in diamond by improving the external quantum efficiency to higher
than 95%. Our work provides a promising route to investigate the laser
cooling effect in diamond.
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