This dissertation details the use of 93Nb (γ = 10.405 MHz/T, I = 9/2) and 77Se (γ = 8.13 MHz/T, I = 1/2) nuclear magnetic resonance
(NMR) to investigate the phase transitions of the layered transition metal dichalcogenide (TMD) niobium diselenide (2H-NbSe2). 2H-NbSe2 has a
trigonal prismatic structure and exhibits a charge density wave (CDW) transition at TCDW = 33.5 K and a superconducting (SC) transition with
Tc = 7.2 K. The present experiments were undertaken with the external field (H0) first parallel, and then perpendicular to the
crystallographic axis (c-axis). Single crystals of 2H-NbSe2 were probed in the temperature range of 0.35 K – 100 K and at frequencies and
fields ranging from 19 MHz – 135 MHz and 1.8 T – 17.5 T respectively to investigate the normal, CDW and superconducting states. A value of Tc
= 7.04 K was measured in situ by cooling the sample below Tc and measuring detuning. A value of Tc = 6.2 K was measured in a sample grown
from the same batch using a SQUID magnetometer. 93Nb NMR spectral line shape and Knight shift were used to detect the CDW phase. The full
93Nb spectrum was field-swept and observed to possess line broadening and asymmetry which gain intensity the further the respective
transition is from the central line. Pre-transitional broadening was detected in both orientations, beginning at a temperature of 60 K for
the central transition and as high as 80 K for the first lower-frequency satellite transition. The broadening continues below TCDW and ends
below 15 K. These results are believed to be evidence of a discommensurate CDW phase. Spin-lattice relaxation rate (T1) measurements are used
to directly probe the electronic density of states (DOS) and investigate the SC gap. The T1 data of both nuclei reveal Korringa behavior
above Tc, no visible Hebel-Slichter peak just below Tc, and a linear crossover to further Korringa behavior for T << Tc in both
orientations. Both orientations were accurately fit to a two-gap function and 1/T1T in the H0⊥c direction displays a two-step transition,
suggesting two superconducting gaps are present. A field-dependence of T1 in the SC state was detected in both sample orientations. In both
the H0∥c and H0⊥c directions, 〖1/T〗_1∝H due to the Zeeman contribution of the Volovik effect. The Doppler effect contribution from the vortex
supercurrents was absent from the relaxation, which suggests 2H-NbSe2 is a two-gap s-wave superconductor. / A Dissertation submitted to the Program in Materials Science and Engineering in partial fulfillment of the
requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / November 14, 2017. / Charge Density Wave, Condensed Matter Physics, NbSe2, NMR, Superconductivity, Transition Metal
Dichalcogenide / Includes bibliographical references. / Arneil Reyes, Professor Co-Directing Dissertation; Irinel Chiorescu, Professor Co-Directing
Dissertation; Adrian Barbu, University Representative; Eric Hellstrom, Committee Member; Theo Siegrist, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_605032 |
| Contributors | Wilson, Douglas M. (Douglas Mark) (author), Reyes, Arneil P. (professor co-directing dissertation), Chiorescu, Irinel (professor co-directing dissertation), Barbu, Adrian G., 1971- (university representative), Hellstrom, Eric (committee member), Siegrist, Theo (committee member), Florida State University (degree granting institution), Graduate School (degree granting college), Program in Materials Science (degree granting departmentdgg) |
| Publisher | Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text, doctoral thesis |
| Format | 1 online resource (132 pages), computer, application/pdf |
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