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Register a new userTetramethylbenzidine-TetrafluoroTCNQ: A narrow-gap semiconducting salt with room temperature relaxor ferroelectric behavior
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We present an extension and revision of the spectroscopic and structural data of the mixed stack charge transfer (CT) crystal 3,3$^prime$,5,5$^prime$-tetramethylbenzidine--tetrafluoro-tetracyanoquinodimethane (TMB-TCNQF$_4$), associated with new electric and dielectric measurements. Refinement of syncrotron structural data at low temperature have led to revise the previously reported [Phys. Rev. Mat. textbf{2}, 024602 (2018)] $C2/m$ structure. The revised structure is $P2_1/m$, with two dimerized stacks per unit cell, and is consistent with the vibrational data. However, polarized Raman data in the low-frequency region also indicate that by increasing temperature above 200 K the structure presents an increasing degree of disorder mainly along the stack axis. This finds confirmation in the analysis of the anisotropic displacement parameters of the structure. TMB-TCNQF$_4$ is confirmed to be a narrow gap semiconductor ($E_a sim 0.3$ eV) with room $T$ conductivity of $sim 10^{-4}~ Omega^{-1}$ cm$^{-1}$, while dielectric measurement have evidenced a typical relaxor ferroelectric behavior already at room $T$, with a peak in real part of dielectric constant $epsilon(T, u)$ around 200 K and 0.1 Hz. The relaxor behavior is explained in terms of the presence of spin solitons separating domains of opposite polarity that yield to ferrolectric nanodomains.
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Keeping current interests to identify materials with intrinsic magnetodielectric behavior near room temperature and with novel pyroelectric current anomalies, we report temperature and magnetic-field dependent behavior of complex dielectric permittivity and pyroelectric current for an oxide, Li2Ni2Mo3O12, containing magnetic ions with (distorted) honey-comb and chain arrangement and ordering magnetically below 8 K. The dielectric data reveal the existence of relaxor ferroelectricity behavior in the range 160-240 K and there are corresponding Raman mode anomalies as well in that temperature range. Pyrocurrent behavior is also consistent with this interpretation, with the pyrocurrent peak-temperature interestingly correlating with the poling temperature. 7Li NMR offer an evidence for crystallographic disorder intrinsic to this compound and we therefore conclude that such a disorder is apparently responsible for the randomness of local electric field leading to relaxor ferroelectric property. Another observation of emphasis is that there is a notable decrease in the dielectric constant with the application of magnetic field to the tune of about -2.4% at 300 K, with the magnitude varying mariginally with temperature. Small loss factor values validate intrinsic behavior of the magnetodielectric effect at room temperature.
New carbon forms exhibiting extraordinary physico-chemical properties can be generated from nanostructured precursors under extreme pressure. Nevertheless, synthesis of such fascinating materials is often not well understood that results, as is the case of C60 precursor, in irreproducibility of the results and impeding further progress in the materials design. Here the semiconducting amorphous carbon having bandgaps of 0.1-0.3 eV and the advantages of isotropic superhardness and superior toughness over single-crystal diamond and inorganic glasses are produced from transformation of fullerene at high pressure and moderate temperatures. A systematic investigation of the structure and bonding evolution was carried out by using rich arsenal of complimentary characterization methods, which helps to build a model of the transformation that can be used in further high p,T synthesis of novel nanocarbon systems for advanced applications. The produced amorphous carbon materials have the potential of demanding optoelectronic applications that diamond and graphene cannot achieve
ABSTRACT: Narrow-gap semiconductors are sought-after materials due to their potential for long-wavelength detectors, thermoelectrics, and more recently non-trivial topology. Here we report the synthesis and characterization of a new family of narrow-gap semiconductors, $R$$_{3}$Cd$_{2}$As$_{6}$ ($R=$ La, Ce). Single crystal x-ray diffraction at room temperature reveals that the As square nets distort and Cd vacancies order in a monoclinic superstructure. A putative charge-density ordered state sets in at 279~K in La$_{3}$Cd$_{2}$As$_{6}$ and at 136~K in Ce$_{3}$Cd$_{2}$As$_{6}$ and is accompanied by a substantial increase in the electrical resistivity in both compounds. The resistivity of the La member increases by thirteen orders of magnitude on cooling, which points to a remarkably clean semiconducting ground state. Our results suggest that light square net materials within a $I4/mmm$ parent structure are promising clean narrow-gap semiconductors.
Here we investigate the thermodynamic and electronic properties of Eu$_{11}$InSb$_9$ single crystals. Electrical transport data show that Eu$_{11}$InSb$_9$ has a semiconducting ground state with a relatively narrow band gap of $320$~meV. Magnetic susceptibility data reveal antiferromagnetic order at low temperatures, whereas ferromagnetic interactions dominate at high temperature. Specific heat, magnetic susceptibility, and electrical resistivity measurements reveal three phase transitions at $T_{N1}=9.3$~K, $T_{N2} =8.3$~K, and $T_{N3} =4.3$~K. Unlike Eu$_{5}$In$_{2}$Sb$_6$, a related europium-containing Zintl compound, no colossal magnetoresistance (CMR) is observed in Eu$_{11}$InSb$_9$. We attribute the absence of CMR to the smaller carrier density and the larger distance between Eu ions and In-Sb polyhedra in Eu$_{11}$InSb$_9$. Our results indicate that Eu$_{11}$InSb$_9$ has potential applications as a thermoelectric material through doping or as a long-wavelength detector due to its narrow gap.
Materials with reduced dimensions have been shown to host a wide variety of exotic properties and novel quantum states that often defy textbook wisdom1-5. Ferroelectric polarization and metallicity are well-known examples of mutually exclusive properties that cannot coexist in bulk solids because the net electric field in a metal can be fully screened by free electrons6. An atomically thin metallic layer capped by insulating layers has shown decent conductivity at room temperature7. Moreover, a penetrating polarization field can be employed to induce an ion displacement and create an intrinsic polarization in the metallic layer. Here we demonstrate that a ferroelectric metal can be artificially synthesized through imposing a strong polarization field in the form of ferroelectric/unit-cell-thin metal superlattices. In this way the symmetry of an atomically thin conductive layer can be broken and manipulated by a neighboring polar field, thereby forming a two-dimensional (2D) ferroelectric metal. The fabricated of (SrRuO3)1/(BaTiO3)10 superlattices exhibit ferroelectric polarization in an atomically thin layer with metallic conductivity at room temperature. A multipronged investigation combining structural analyses, electrical measurements, and first-principles electronic structure calculations unravels the coexistence of 2D electrical conductivity in the SrRuO3 monolayer accompanied by the electric polarization. Such 2D ferroelectric metal paves a novel way to engineer a quantum multi-state with unusual coexisting properties, such as ferroelectrics, ferromagnetics and metals, manipulated by external fields8,9.
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