335 research outputs found
The NMDA receptor GluN2C subunit controls cortical excitatoryinhibitory balance, neuronal oscillations and cognitive function
Despite strong evidence for NMDA receptor (NMDAR) hypofunction as an underlying factor for cognitive disorders, the precise roles of various NMDAR subtypes remains unknown. The GluN2Ccontaining NMDARs exhibit unique biophysical properties and expression pattern, and lower expression of GluN2C subunit has been reported in postmortem brains from schizophrenia patients. We found that loss of GluN2C subunit leads to a shift in cortical excitatory-inhibitory balance towards greater inhibition. Specifically, pyramidal neurons in the medial prefrontal cortex (mPFC) of GluN2C knockout mice have reduced mEPSC frequency and dendritic spine density and a contrasting higher frequency of mIPSCs. In addition a greater number of perisomatic GAD67 puncta was observed suggesting a potential increase in parvalbumin interneuron inputs. At a network level the GluN2C knockout mice were found to have a more robust increase in power of oscillations in response to NMDAR blocker MK- 801. Furthermore, GluN2C heterozygous and knockout mice exhibited abnormalities in cognition and sensorimotor gating. Our results demonstrate that loss of GluN2C subunit leads to cortical excitatoryinhibitory imbalance and abnormal neuronal oscillations associated with neurodevelopmental disorders
Two Band Model Interpretation of the p to n Transition in Ternary Tetradymite Topological Insulators
The requirement for large bulk resistivity in topological insulators has led
to the design of complex ternary and quaternary phases with balanced donor and
acceptor levels. A common feature of the optimized phases is that they lie
close to the p to n transition. The tetradymite Bi2Te3_xSex system exhibits
minimum bulk conductance at the ordered composition Bi2Te2Se. By combining
local and integral measurements of the density of states, we find that the
point of minimum electrical conductivity at x=1.0 where carriers change from
hole-like to electron-like is characterized by conductivity of the mixed type.
Our experimental findings, which are interpreted within the framework of a two
band model for the different carrier types, indicate that the mixed state
originates from different type of native defects that strongly compensate at
the crossover point
High Thermoelectric Performance in PbSeâNaSbSe2 Alloys from Valence Band Convergence and Low Thermal Conductivity
PbSe is an attractive thermoelectric material due to its favorable electronic structure, high melting point, and lower cost compared to PbTe. Herein, the hitherto unexplored alloys of PbSe with NaSbSe2 (NaPbmSbSem+2) are described and the most promising pâtype PbSeâbased thermoelectrics are found among them. Surprisingly, it is observed that below 500 K, NaPbmSbSem+2 exhibits unorthodox semiconductingâlike electrical conductivity, despite possessing degenerate carrier densities of â1020 cmâ3. It is shown that the peculiar behavior derives from carrier scattering by the grain boundaries. It is further demonstrated that the high solubility of NaSbSe2 in PbSe augments both the thermoelectric properties while maintaining a rock salt structure. Namely, density functional theory calculations and photoemission spectroscopy demonstrate that introduction of NaSbSe2 lowers the energy separation between the Lâ and ÎŁâvalence bands and enhances the power factors under 700 K. The crystallographic disorder of Na+, Pb2+, and Sb3+ moreover provides exceptionally strong point defect phonon scattering yielding low lattice thermal conductivities of 1â0.55 W mâ1 Kâ1 between 400 and 873 K without nanostructures. As a consequence, NaPb10SbSe12 achieves maximum ZT â1.4 near 900 K when optimally doped. More importantly, NaPb10SbSe12 maintains high ZT across a broad temperature range, giving an estimated record ZTavg of â0.64 between 400 and 873 K, a significant improvement over existing pâtype PbSe thermoelectrics.The high solubility of NaSbSe2 in PbSe is exploited to facilitate convergence of Lâ and ÎŁâvalence bands and to produce strong point defect phonon scattering. These processes yield enhanced power factors and low lattice thermal conductivity over â300â700 K, which together give NaPb10SbSe12 outstanding thermoelectric performance with a maximum ZT â 1.4 at 873 K and ZTavg â0.64 over 400â873 K.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151353/1/aenm201901377.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151353/2/aenm201901377-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151353/3/aenm201901377_am.pd
High Thermoelectric Performance in Supersaturated Solid Solutions and Nanostructured nâ Type PbTeâ GeTe
Sbâ doped and GeTeâ alloyed nâ type thermoelectric materials that show an excellent figure of merit ZT in the intermediate temperature range (400â 800 K) are reported. The synergistic effect of favorable changes to the band structure resulting in high Seebeck coefficient and enhanced phonon scattering by point defects and nanoscale precipitates resulting in reduction of thermal conductivity are demonstrated. The samples can be tuned as singleâ phase solid solution (SS) or twoâ phase system with nanoscale precipitates (Nano) based on the annealing processes. The GeTe alloying results in band structure modification by widening the bandgap and increasing the densityâ ofâ states effective mass of PbTe, resulting in significantly enhanced Seebeck coefficients. The nanoscale precipitates can improve the power factor in the low temperature range and further reduce the lattice thermal conductivity (ĂÂşlat). Specifically, the Seebeck coefficient of Pb0.988Sb0.012Teâ 13%GeTeâ Nano approaches â 280 ĂÂľV Kâ 1 at 673 K with a low ĂÂşlat of 0.56 W mâ 1 Kâ 1 at 573 K. Consequently, a peak ZT value of 1.38 is achieved at 623 K. Moreover, a high average ZTavg value of â 1.04 is obtained in the temperature range from 300 to 773 K for nâ type Pb0.988Sb0.012Teâ 13%GeTeâ Nano.Both supersaturated solid solutions and nanostructured nâ type Pb1â xGexTe systems with excellent thermoelectric performance can be prepared via a nonequilibrium process. The nanostructured sample enhances the figure of merit ZT via reducing the lattice thermal conductivity. A ZTavg of â 1.04 is obtained, which is among the highest ZTavg values for nâ type PbTe materials reported so far.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145314/1/adfm201801617-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145314/2/adfm201801617.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145314/3/adfm201801617_am.pd
Ultralow Thermal Conductivity, Multiband Electronic Structure and High Thermoelectric Figure of Merit in TlCuSe
The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W mâ1 Kâ1), a high power factor (11.6 ÂľW cmâ1 Kâ2), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe4 tetrahedra and long TlSe bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl+ lone-pair electrons, boosts phononâphonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl+ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Î-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials
- âŚ