775 research outputs found
NMR relaxation studies of electronic structure in NbSe3
NMR spin-lattice relaxation measurements of the Nb93 resonance for each Nb site were performed on an aligned, multicrystalline NbSe3 sample at different temperatures. Results are associated with local electron densities of states for each of the three crystallographic sites, demonstrating Fermi-surface changes associated with the two charge-density-wave phase transitions. The most significant Fermi-surface changes occur for the yellow and orange crystallographic sites, at the high- and low-temperature phase transitions, respectively. The third site, however, is found to be noninsulating. A comparison is made to band theory and other experimental results
Topological nodal line in ZrTe demonstrated by nuclear magnetic resonance
In this work, we report nuclear magnetic resonance (NMR) combined with
density functional theory (DFT) studies of the transition metal dichalcogenide
ZrTe. The measured NMR shift anisotropy reveals a quasi-2D behavior
connected to a topological nodal line close to the Fermi level. With the
magnetic field perpendicular to the ZrTe layers, the measured shift can be
well-fitted by a combination of enhanced diamagnetism and spin shift due to
high mobility Dirac electrons. The spin-lattice relaxation rates with external
field both parallel and perpendicular to the layers at low temperatures match
the expected behavior associated with extended orbital hyperfine interaction
due to quasi-2D Dirac carriers. In addition, calculated band structures also
show clear evidence for the existence of nodal line in ZrTe between
and A. For intermediate temperatures, there is a sharp reduction in
spin-lattice relaxation rate which can be explained as due to a reduced
lifetime for these carriers, which matches the reported large change in
mobility in the same temperature range. Above 200 K, the local orbital
contribution starts to dominate in an orbital relaxation mechanism revealing
the mixture of atomic functions.Comment: 9 pages, 5 figure
Dirac electron behavior and NMR evidence for topological band inversion in ZrTe5
We report Te NMR measurements of the topological quantum material
ZrTe. Spin-lattice relaxation results, well-explained by a theoretical
model of Dirac electron systems, reveal that the topological characteristic of
ZrTe is -dependent, changing from weak topological insulator to strong
topological insulator as temperature increases. Electronic structure
calculations confirm this ordering, the reverse of what has been proposed. NMR
results demonstrate a gapless Dirac semimetal state occurring at a Lifshitz
transition temperature, K in our crystals. We demonstrate that the
changes in NMR shift at also provide direct evidence of band inversion
when the topological phase transition occurs.Comment: 5 pages, 4 figure
Gap-opening transition in Dirac semimetal ZrTe
We apply Te nuclear magnetic resonance (NMR) spectroscopy to
investigate the Dirac semimetal ZrTe. With the NMR magnetic field parallel
to the -axis, we observe significant quantum magnetic effects. These include
an abrupt drop at 150 K in spin-lattice relaxation rate. This corresponds to a
gap-opening transition in the Dirac carriers, likely indicating the onset of
excitonic pairing. Below 50 K, we see a more negative shift for the Te
bridging site indicating the repopulation of Dirac levels with spin polarized
carriers at these temperatures. This is the previously reported 3D quantum Hall
regime; however, we see no sign of a charge density wave as has been proposed.Comment: 5 pages, 4 figure
Direct Measure of Giant Magnetocaloric Entropy Contributions in Ni-Mn-In
Off-stoichiometric alloys based on Ni 2 MnIn have drawn attention due to the
coupled first order magnetic and structural transformations, and the large
magnetocaloric entropy associated with the transformations. Here we describe
calorimetric and magnetic studies of four compositions. The results provide a
direct measure of entropy changes contributions including at the first-order
phase transitions, and thereby a determination of the maximum field-induced
entropy change corresponding to the giant magnetocaloric effect. We find a
large excess entropy change, attributed to magneto-elastic coupling, but only
in compositions with no ferromagnetic order in the high-temperature austenite
phase. Furthermore, a molecular field model corresponding to antiferromagnetism
of the low-temperature phases is in good agreement, and nearly independent of
composition, despite significant differences in overall magnetic response of
these materials
Superconductivity and Magnetism in Silicon and Germanium Clathrates
Clathrates are materials containing closed polyhedral cages stacked to form crystalline frameworks. With Si, Ge, and Sn atoms populating these frameworks, a wide variety of electronic and vibrational properties can be produced in these materials, by substitution upon framework sites or through incorporation of ions in cage-center positions. Commonly-formed structures include the type I, type II, and chiral clathrate types, whose properties will be described here. Ba8Si46 with the type-I structure has been found to exhibit superconductivity with Tc as high as 9 K. The enhanced Tc in this compound has been shown to arise predominantly from very sharp features in the electronic densities of states associated with the extended sp3- bonded framework. Atomic substitution can tailor these electronic properties, however the associated disorder has been found to inevitably lower the Tc due to the disrupted continuity of the framework. Efforts to produce analogous Ge-based superconductors have not been successful, due to the appearance of spontaneous vacancies, which also serve to disrupt the frameworks. The formation of these vacancies is driven by the Zintl mechanism, which plays a much more significant role for the structural stability of the Ge clathrates. The sharp density of states features in these extended framework materials may also lead to enhanced magnetic features, due to conduction-electron mediated coupling of substituted magnetic ions. This has led to magnetic ordering in Fe and Mn-substituted clathrates. The largest number of clathrates exhibiting magnetic behavior has been produced by substitution of Eu on cage-center sites, with a ferromagnetic Tc as high as 38 K observed in such materials
NMR Study of Ba8Cu5SixGe41-x Clathrate Semiconductors
We have performed 63Cu, 65Cu, and 137Ba NMR on Ba8Cu5SixGe41-x, a series of intermetallic clathrates known for their potential as thermoelectric materials, in order to investigate the electronic behavior of the samples. The spectra and spin - lattice relaxation times were measured at 77 K and 290 K for the entire composition range 0 <= x <= 41. Magnetic and quadrupole shifts and relaxation rates of the Cu NMR data were extracted, and thereby carrier-induced metallic contributions identified. The observed shifts change in a nonlinear way with increasing Si substitution: from x = 0 to about 20 the shifts are essentially constant, while approaching x = 41 they increase rapidly. At the same time, Ba NMR data indicate greater Ba-site participation in the conduction band in Ba8Cu5Si41 than in Ba8Cu5Ge41. The results indicate surprisingly little change in electronic features vs. Si content for most of the composition range, while Ba8Cu5Si41 exhibits enhanced hybridization and a more metallic framework than Ba8Cu5Ge41.Robert A. Welch Foundation, Grant No. A-1526.
Austrian Science Fund, FWF project TRP 176-N22
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