93 research outputs found
Electronic Instability and Anharmonicity in SnSe
The binary compound SnSe exhibits record high thermoelectric performance,
largely because of its very low thermal conductivity. The origin of the strong
phonon anharmonicity leading to the low thermal conductivity of SnSe is
investigated through first-principles calculations of the electronic structure
and phonons. It is shown that a Jahn-Teller instability of the electronic
structure is responsible for the high-temperature lattice distortion between
the Cmcm and Pnma phases. The coupling of phonon modes and the phase transition
mechanism are elucidated, emphasizing the connection with hybrid improper
ferroelectrics. This coupled instability of electronic orbitals and lattice
dynamics is the origin of the strong anharmonicity causing the ultralow thermal
conductivity in SnSe. Exploiting such bonding instabilities to generate strong
anharmonicity may provide a new rational to design efficient thermoelectric
materials
Orbital-Energy Splitting in Anion Ordered Ruddlesden-Popper Halide Perovskites for Tunable Optoelectronic Applications
The electronic orbital characteristics at the band edges plays an important
role in determining the electrical, optical and defect properties of perovskite
photovoltaic materials. It is highly desirable to establish the relationship
between the underlying atomic orbitals and the optoelectronic properties as a
guide to maximize the photovoltaic performance. Here, using first-principles
calculations and taking anion ordered Ruddlesden-Popper (RP) phase halide
perovskites CsGeICl as an example, we demonstrate
how to rationally optimize the optoelectronic properties (e.g., band gap,
transition dipole matrix elements, carrier effective masses, band width)
through a simple band structure parameter. Our results show that reducing the
splitting energy of p orbitals of B-site atom can effectively
reduce the band gap and carrier effective masses while greatly improving the
optical absorption in the visible region. Thereby, the orbital-property
relationship with is well established through biaxial compressive
strain. Finally, it is shown that this approach can be reasonably extended to
several other non-cubic halide perovskites with similar p orbitals
characteristics at the conduction band edges. Therefore, we believe that our
proposed orbital engineering approach provides atomic-level guidance for
understanding and optimizing the device performance of layered perovskite solar
cells
External uniform electric field removing flexoelectric effect in epitaxial ferroelectric thin films
Using the modified Landau-Ginsburg-Devonshire thermodynamic theory, it is
found that the coupling between stress gradient and polarization, or
flexoelectricity, has significant effect on ferroelectric properties of
epitaxial thin films, such as polarization, free energy profile and hysteresis
loop. However, this effect can be completely eliminated by applying an
optimized external, uniform electric field. The role of such uniform electric
field is shown to be the same as that of an ideal gradient electric field which
can suppress the flexoelectricty effect completely based on the present theory.
Since the uniform electric field is more convenient to apply and control than
gradient electric field, it can be potentially used to remove the flexoelectric
effect induced by stress gradient in epitaxial thin films and enhance the
ferroelectric properties.Comment: 5 pages, 3 figure
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