Nov 25, U.S. Marine Corps Supply administration and operations specialists (MOS ) perform technical duties in retail and wholesale supply. Read Online Now mos roadmap Ebook PDF at our Library. Get mos roadmap PDF file for free from our online library. PDF File: mos roadmap. Dec 18, MOS Roadmaps. Marine Corps Training Information Management System ( MCTIMS) Roadmaps. CAC login or separate account required.
|Country:||United Arab Emirates|
|Published (Last):||25 January 2012|
|PDF File Size:||1.25 Mb|
|ePub File Size:||15.27 Mb|
|Price:||Free* [*Free Regsitration Required]|
a program of MarineParents.com
Electrostatic doping techniques giving access to high carrier densities are needed to achieve such phase transitions. Contrary to earlier reports, after the onset of superconductivity, the superconducting transition temperature does not depend on the carrier density. Our doping method and the results we obtain in MoS 2 for samples as thin as bilayers indicates the potential of this approach. Layered materials are being extensively studied for their exceptional properties when reduced to few atomic layers.
The promise of producing innovative electronic devices is alluring. So is the perspective of electrostatic doping to induce substantial carrier density and phase transitions 1 in two-dimensions 2D. Recent research in both fields has widely used molybdenum disulphide MoS 2 as a prototype.
Bulk MoS 2 is a semiconductor with an indirect electronic band-gap. The strong light—matter interaction at these points 4 and the lifting of the spin degeneracy of the valence and conduction bands due to inversion symmetry breaking make single layer MoS 2 an excellent material for transistors in electronics, opto-electronics and the developing valleytronics field 5678. Chemical intercalation can induce an n -doped metallic and superconducting state 910a fact that has inspired electrostatic and electrochemical doping of few-layer MoS 2.
Both these techniques involve delicate device issues and whereas a solid dielectric does not allow for very high doping, the liquid gating method requires great care to avoid possible intercalation, electrochemical reactions 16 and strain during the liquid to solid transformation of the dielectric, which may be avoided by using a solid ionic conductor A layered sample is placed on the substrate and an electric field induces an ionic drift and a space charge at the sample-substrate interface leading to a very strong electrostatic field.
Applying a positive resp. This space charge at the interface induces electrostatic doping of the few-layer sample. Thus, by applying a positive or negative gate voltage of the order of 1— V, the sample can be doped ambivalently with a consequent variation of its resistivity, which can be monitored along with the gate current as a function of time. The carrier density that depends on the total charge transferred during the doping process is determined in this static state by Hall effect measurements.
Heating of the polarized substrate causes a drift current which tends to nullify the space charge layer and thus remove electrostatic doping of the sample.
This process can be accelerated by the application of an appropriate electric field. Raman spectra of samples before and after doping cycles do not show any signs of modification Supplementary Fig. In this study, we show the result of space charge doping applied to few-layer MoS 2 with varying thickness. An insulator—metal transition is induced at moderate doping.
The very high carrier density accessible with our doping technique allows us to subsequently induce superconductivity.
MoS 2 samples are prepared and contacted as described in the Methods section. Transport measurements are performed in a high vacuum cryostat coupled to an external 2-T electromagnet for Hall measurements. The properties of MoS 2 have been shown to be particularly sensitive to adsorbates, especially in the few-layer limit 2324explaining this observation. The annealing also ensures the removal of any residual space charge from the sample fabrication step. We show results from three samples with varying thickness: An example of the space charge doping procedure is shown in Fig.
The insulator—metal transition occurring in the low to moderate carrier density range 0. Mow the doping dynamics is slow and electrostatic equilibrium is never reached in our experiments Fig. Indeed, the same carrier density can be obtained with a lower gate voltage but longer time, as what 3034 is the accumulated space charge.
The temperature is shown as the red curve right axisthe vertical dash line marks the limit between the doping itself and the following temperature quench.
current protocols pdf ebook download
The samples 11, 4. In the inset of Fig. This behaviour originates from quantum interference corrections to the conductivity electrons are back-scattered coherently after multiple collisions and thus is only seen at low temperature, while the metal—insulator transition corresponding to the quantum of resistance originates from strong localization of electronic wave-functions the mean free path is shorter than the electronic wavelength and the electrons do not propagate.
It is thus seen at much higher temperatures K. The transition can be reversed by hole doping with a negative gate voltage which pushes the sample back into the insulating state.
The carrier densities are measured by Hall effect. All the measured Hall resistances R H B show linear behaviour with the magnetic field in the range we used Fig. The carrier concentration decreases with temperature as shown in Fig. The cation mobility in the glass being negligible below K, this effect is not due to a loss of polarization in the glass and has been verified to depend only on temperature and not on applied gate voltage.
Military Occupational Specialty Location Database
These traps can capture electrons from the MoS 2 sample at low temperature, thus reducing the carrier density, while at high temperatures they are ionized resulting in more efficient space charge doping.
Carrier densities are extracted directly from the Hall coefficient assuming a single band with isotropic mass and diffusion coefficient. High doping could involve carriers in multiple valleys with different band mass because of the complex structure of the MoS 2 conduction band A roadmwp model assuming two bands implies non-linear Hall resistance behaviour not seen in our measurements.
Black dashed lines are linear fits. For the 11 nm sample the red arrows pointing to the right are measurements made while increasing the carrier density, while the purple arrows pointing to the left are made while decreasing carrier density after maximum doping.
Dashed lines are guides for the eye. The mobilities derived from the Hall carrier density at high temperature K are shown in Fig. At low carrier density, the mobility increases with carrier density as it has been reported in solid states FET devices 12whereas the high density mobility is limited by mps scattering processes 3031 There is also the possibility that the electrostatic pressure at very high doping may locally deform the crystal lattice to adapt to the roughness of the amorphous glass substrate.
However, MoS 2 nanosheets can be deformed reversibly to several nanometres parallel to c roaddmap 34while the roughness of our substrates has been measured to be between 0. Thus the possibility of irreversible deformation is small. Accordingly, mobility before and after maximum doping for the 11 nm sample does not change as shown in Fig. At low temperatures 4. These values are in agreement with previous reports on MoS 2 in these doping regimes 35 The differences in roaxmap in the three samples are most likely related to the quality of the initial bulk samples.
Beyond the insulator to metallic transition which occurs at moderate doping, the samples were doped to higher carrier densities. The maximum carrier densities measured at room temperature were 1.
The much higher value obtained for the 11 nm sample is explained by the fact that it is placed on glass with a higher sodium concentration soda-lime instead of borosilicate glass. At the relatively low maximum carrier density achieved in the 4. Figure roadkap shows the measurements on the 11 nm sample that was first doped to the roqdmap carrier density where a sharp fall in resistance at low temperatures, indicating superconductivity, appeared.
Doping was then progressively decreased resulting in the expected monotonic increase in the room temperature sheet resistance and eventual disappearance of superconductivity as shown in Fig.
The low temperature sheet resistance evolves in a more complicated manner with decreasing doping. A signature of the superconducting transition is roadmzp dependence on the applied perpendicular magnetic field. As shown in Fig. This well known variation of the transition temperature is moa linear close to T C as is observed for the onset superconducting temperature in our sample in the inset of Fig. The arrow indicates the sequence of measurements. Critical field B Rooadmap as a function of onset critical temperature, with linear fit roadmmap line.
The error bars relate to the extraction of the onset jos temperature. The increase in sheet resistance at the highest doping levels is probably caused by irreversible crystalline deformation in this ultra-thin sample as discussed above. Both samples thus display an incomplete low temperature superconducting transition with significant differences from the one reported in ref. First, it is incomplete with non-zero resistance below T C as can be seen in Fig. This implies the existence of a dissipating normal phase at temperatures lower than T C defined as the superconducting onset temperature.
Second, each sample shows this behaviour over doping ranges that are different from those reported earlier. Though electrostatic doping is probably confined to a thickness corresponding to less than the bilayer in both samples, the higher critical temperature in the thicker sample could be due to a c axis superconducting coherence length in superconducting MoS 2 that is greater than the bilayer thickness.
CURRENT PROTOCOLS PDF
Even in the insulating regime the conductivity shows a transition from 2D to 3D behaviour in going from the bilayer to the thick sample Supplementary Note 2.
Critical temperature corresponding to the onset of the superconducting transition as a function of Hall carrier density measured at low temperature for the 2 nm red circles and the 11 nm black circles samples. Open symbols represent values reported in ref. Residual fermions continue to localize below T C.
As a cross-check on the reproducibility of our results all three samples were measured at least twice. Between measurements each sample was reduced to its original post-annealing doping state and subsequently removed from the cryostat, measured with Raman spectroscopy and stored in a primary vacuum for days or weeks.
Reintroduction into the cryostat followed by doping cycles and measurement gave reproducible results. Non-zero resistance in superconducting materials is an ongoing matter of research as multiple factors could lead to the appearance of dissipation above and below the superconducting transition, especially in low dimensions. Indeed, thermal vortex generation produces dissipation at any non-zero temperature below T C in a 2D superconducting strip 37and the finite size of the sample may limit the low temperature divergence of the coherence length of the BKT transition leading to a vanishing resistive tail at low temperature On the other hand, non percolating superconducting phases can display saturating non-zero resistance at zero temperature in amorphous systems 39or systems with inhomogeneities at low carrier densities As they are produced by exfoliation, our samples are single crystals with low defect density as shown by the measured mobilities.
However, the saturation resistivity shown by the 11 nm sample is too high to be explained by vortices or finite size effects alone. The space charge layer in the amorphous glass substrate may also display inhomogeneities at the nanometre scale. The resulting doping inside the crystalline MoS 2however, should be smoothed out by Coulomb screening, especially at high doping. Finally, the non-zero resistance could be attributed to some other extrinsic factor arising from our unique method.
Preliminary measurements on a monolayer high critical temperature superconductor sample show that zero resistance superconductivity is attained in this sample under similar conditions, putting to doubt this last possibility. The saturation resistivity of the 11 nm sample even at the highest carrier densities is thus intriguing and will require further investigation.
The independence of T C on carrier density Fig. However, results similar to ours have recently been reported in ionic liquid doped WS 2 ref. BCS superconductivity and the corresponding T C depend on phonon frequencies, electron—phonon coupling and the density of states DOS at the Fermi level.