Abstract
We report the influence on the superconducting critical temperature
T
c
in doped SrTiO
3
of the substitution of the natural
16
O atoms by the heavier isotope
18
O. We observe that for a wide ...range of doping this substitution causes a strong (~50%) enhancement of
T
c
. Also the magnetic critical field
H
c
2
is increased by a factor ~2. Such a strong impact on
T
c
and
H
c
2
, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO
3
.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which ...have not been fully addressed. WSe2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E', E″ and E2g modes shift to lower frequencies, and the A'1 and A1g modes shift to higher frequencies. Second, new high frequency first order A'1 and A1g modes appear, explaining recently reported experimental data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A'1 and A1g is found which explains those observed in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Abstract
Studying the collective excitations in charge neutral graphene (CNG) has recently attracted a great interest because of unusual mechanisms of the charge carrier dynamics. The latter can play ...a crucial role for formation of recently observed in twisted bilayer CNG graphene plasmon polaritons (GPPs) associated with the interband transitions between the flat electronic bands. Besides, GPPs in CNG can be a tool providing insights into various quantum phenomena in CNG via optical experiments. However, the properties of interband GPPs in CNG are not known, even in the simplest configurations. Here, we show that magnetically-biased single-layer CNG can support interband GPPs of both transverse magnetic and transverse electric polarizations (particularly, at zero temperature). GPPs exist inside the absorption bands originating from the electronic transitions between Landau levels and are tunable by the magnetic field. We place our study into the context of potential near-field and far-field optical experiments.
Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2. Without dielectric ...engineering and beyond a T-dependent threshold gate-voltage, we observe maximum hole mobilities approaching 350 cm(2)/Vs at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm(2)/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2. We argue that improvements in the fabrication protocols as, for example, the use of a substrate free of dangling bonds are likely to produce WSe2-based FETs displaying higher room temperature mobilities, i.e. approaching those of p-doped Si, which would make it a suitable candidate for high performance opto-electronics.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
By using high-magnetic fields (up to 60 T), we observe compelling evidence of the integer quantum Hall effect in trilayer graphene. The magnetotransport fingerprints are similar to those of the ...graphene monolayer, except for the absence of a plateau at a filling factor of ν=2. At a very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder. The measured Hall resistivity plateaus are well reproduced theoretically, using a self-consistent Hartree calculations of the Landau levels and assuming an ABC stacking order of the three layers.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Using transport measurements, we investigate multicomponent quantum Hall (QH) ferromagnetism in dual-gated rhombohedral trilayer graphene (r-TLG) in which the real spin, orbital pseudospin, and layer ...pseudospins of the lowest Landau level form spontaneous ordering. We observe intermediate QH plateaus, indicating a complete lifting of the degeneracy of the zeroth Landau level (LL) in the hole-doped regime. In charge neutral r-TLG, the orbital degeneracy is broken first, and the layer degeneracy is broken last and only in the presence of an interlayer potential U ⊥. In the phase space of U ⊥ and filling factor ν, we observe an intriguing “hexagon” pattern, which is accounted for by a model based on crossings between symmetry-broken LLs.
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Many physical phenomena can be understood by single-particle physics; that is, treating particles as non-interacting entities. When this fails, many-body interactions lead to spontaneous symmetry ...breaking and phenomena such as fundamental particles' mass generation, superconductivity and magnetism. Competition between single-particle and many-body physics leads to rich phase diagrams. Here we show that rhombohedral-stacked trilayer graphene offers an exciting platform for studying such interplay, in which we observe a giant intrinsic gap ~42 meV that can be partially suppressed by an interlayer potential, a parallel magnetic field or a critical temperature ~36 K. Among the proposed correlated phases with spatial uniformity, our results are most consistent with a layer antiferromagnetic state with broken time reversal symmetry. These results reflect the interplay between externally induced and spontaneous symmetry breaking whose relative strengths are tunable by external fields, and provide insight into other low-dimensional systems.
We report on magneto-transport measurement in disordered graphene under a pulsed magnetic field of up to 57 T. For large electron or hole doping, the system displays the expected anomalous integer ...quantum Hall effect (IQHE) specific to graphene down to the filling factor nu = 2. In the close vicinity of the charge neutrality point, the system breaks up into coexisting puddles of holes and electrons, leading to a vanishing Hall and finite longitudinal resistance with no hint of divergence at very high magnetic field. Large resistance fluctuations are observed near the Dirac point. They are interpreted as the natural consequence of the presence of electron and hole puddles. The magnetic field at which the amplitude of the fluctuations is the largest is directly linked to the mean size of the puddles.
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