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Register a new userCleaving-temperature dependence of layered-oxide surfaces
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The surfaces generated by cleaving non-polar, two-dimensional oxides are often considered to be perfect or ideal. However, single particle spectroscopies on Sr2RuO4, an archetypal non-polar two dimensional oxide, show significant cleavage temperature dependence. We demonstrate that this is not a consequence of the intrinsic characteristics of the surface: lattice parameters and symmetries, step heights, atom positions, or density of states. Instead, we find a marked increase in the density of defects at the mesoscopic scale with increased cleave temperature. The potential generality of these defects to oxide surfaces may have broad consequences to interfacial control and the interpretation of surface sensitive measurements.
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The Luttinger surface of an organic metal (TTF-TCNQ), possessing charge order and spin-charge separation, is investigated using temperature dependent angle-resolved photoemission spectroscopy. The Luttinger surface topology, obtained from momentum distribution curves, changes from quasi-2D(dimensional) to quasi-1D with temperature. The high temperature quasi-2D surface exhibits 4$k_F$ charge-density-wave (CDW) superstructure in the TCNQ derived holon band, in the absence of 2$k_F$ order. Decreasing temperature results in quasi-1D nested 2$k_F$ CDW order in the TCNQ spinon band and in the TTF surface. The results establish the link in momentum-space between charge order and spin-charge separation in a Luttinger liquid.
The nature of a puzzling high temperature ferromagnetism of doped mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al., Nature 431 (2004) 672, has been addressed by static magnetization, muon spin relaxation, nuclear magnetic and electron spin resonance spectroscopy techniques. A precise control of the charge doping was achieved by electrochemical Li intercalation. We find that it provides excess electrons, thereby increasing the number of interacting magnetic vanadium sites, and, at a certain doping level, yields a ferromagnetic-like response persisting up to room temperature. Thus we confirm the surprising previous results on the samples prepared by a completely different intercalation method. Moreover our spectroscopic data provide first ample evidence for the bulk nature of the effect. In particular, they enable a conclusion that the Li nucleates superparamagnetic nanosize spin clusters around the intercalation site which are responsible for the unusual high temperature ferromagnetism of vanadium oxide nanotubes.
High mobility two-dimensional electron gases (2DEGs) underpin todays silicon based devices and are of fundamental importance for the emerging field of oxide electronics. Such 2DEGs are usually created by engineering band offsets and charge transfer at heterointerfaces. However, in 2011 it was shown that highly itinerant 2DEGs can also be induced at bare surfaces of different transition metal oxides where they are far more accessible to high resolution angle resolved photoemission (ARPES) experiments. Here we review work from this nascent field which has led to a systematic understanding of the subband structure arising from quantum confinement of highly anisotropic transition metal d-states along different crystallographic directions. We further discuss the role of different surface preparations and the origin of surface 2DEGs, the understanding of which has permitted control over 2DEG carrier densities. Finally, we discuss signatures of strong many-body interactions and how spectroscopic data from surface 2DEGs may be related to the transport properties of interface 2DEGs in the same host materials.
The temperature dependence of the Mott metal-insulator transition (MIT) is studied with a VO_2-based two-terminal device. When a constant voltage is applied to the device, an abrupt current jump is observed with temperature. With increasing applied voltages, the transition temperature of the MIT current jump decreases. We find a monoclinic and electronically correlated metal (MCM) phase between the abrupt current jump and the structural phase transition (SPT). After the transition from insulator to metal, a linear increase in current (or conductivity) is shown with temperature until the current becomes a constant maximum value above T_{SPT}=68^oC. The SPT is confirmed by micro-Raman spectroscopy measurements. Optical microscopy analysis reveals the absence of the local current path in micro scale in the VO_2 device. The current uniformly flows throughout the surface of the VO_2 film when the MIT occurs. This device can be used as a programmable critical temperature sensor.
The phase offset of quantum oscillations is commonly used to experimentally diagnose topologically non-trivial Fermi surfaces. This methodology, however, is inconclusive for spin-orbit-coupled metals where $pi$-phase-shifts can also arise from non-topological origins. Here, we show that the linear dispersion in topological metals leads to a $T^2$-temperature correction to the oscillation frequency that is absent for parabolic dispersions. We confirm this effect experimentally in the Dirac semi-metal Cd$_3$As$_2$ and the multiband Dirac metal LaRhIn$_5$. Both materials match a tuning-parameter-free theoretical prediction, emphasizing their unified origin. For topologically trivial Bi$_2$O$_2$Se, no frequency shift associated to linear bands is observed as expected. However, the $pi$-phase shift in Bi$_2$O$_2$Se would lead to a false positive in a Landau-fan plot analysis. Our frequency-focused methodology does not require any input from ab-initio calculations, and hence is promising for identifying correlated topological materials.
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