We report the observation of a new fractional quantum Hall state in the second Landau level of a two-dimensional electron gas at the Landau level filling factor ν=2+6/13. We find that the model of noninteracting composite fermions can explain the magnitude of gaps of the prominent 2+1/3 and 2+2/3 states. The same model fails, however, to account for the gaps of the 2+2/5 and the newly observed 2+6/13 states suggesting that these two states are of exotic origin.

We report an unexpected sharp peak in the temperature dependence of the magnetoresistance of the reentrant integer quantum Hall states in the second Landau level. This peak defines the onset temperature of these states. We find that in different spin branches the onset temperatures of the reentrant states scale with the Coulomb energy. This scaling provides direct evidence that Coulomb interactions play an important role in the formation of these reentrant states evincing their collective nature.The second Landau level (SLL) of the two-dimensional electron gas (2DEG) is astonishingly rich in novel ground states [1][2][3]. Recent experiments [3][4][5][6][7][8][9] suggest that there are both conventional [10,11] as well as exotic fractional quantum Hall states (FQHSs) [12,13] in this region. The study of the latter has enriched quantum many-body physics with numerous novel concepts such as paired composite fermion states with Pfaffian correlations, non-Abelian quasiparticles [12][13][14][15][16][17][18][19], topologically protected quantum computing [20], and established connections between the 2DEG and p-wave superconductivity in Sr 2 RuO 4 and fermionic atomic condensates.The eight reentrant integer quantum Hall states (RIQHSs) form another set of prominent ground states in the SLL [1]. The transport signatures of the RIQHSs are consistent with electron localization in the topmost energy level [1]. However, the nature of the localization is not yet well understood. Depending on the relative importance of the electron-electron interactions, the ground state can be either an Anderson insulator or a collectively pinned electron solid.FQHSs owe their existence to the presence of the interelectronic Coulomb interactions [10,11]. Since FQHSs and RIQHSs alternate in the SLL, it was argued that Coulomb interactions must be important and, therefore, the RIQHSs in the SLL must be electron solids [1]. Subsequent density matrix renormalization group [21] and Hartree-Fock calculations [22] also favored the electron solid picture and predicted the solid phase similar to the Wigner crystal, but having one or more electrons in the nodes of the crystal [22]. Recently reported weak microwave resonances in one such RIQHS are suggestive of but are far from being conclusive on the formation of a collective insulator [23]. Our understanding of the RIQHSs in the SLL, therefore, is still in its infancy and the collective nature of these states has not yet been firmly established.We report a feature in the temperature dependent magnetoresistance unique to the the RIQHSs in the SLL, a feature which is used to define the onset temperature of these states. The scaling of onset temperatures with the Coulomb energy reveals that Coulomb interactions play a central role in the formation of RIQHSs and, therefore, these reentrant states are exotic electronic solids rather than Anderson insulators. We also report an unexpected trend of the onset temperatures within each spin branch. This trend is inconsistent with current theories and can be unde...

We fabricated a He-3 immersion cell for transport measurements of semiconductor nanostructures at ultra low temperatures and in strong magnetic fields. We have a new scheme of field-independent thermometry based on quartz tuning fork Helium-3 viscometry which monitors the local temperature of the sample's environment in real time. The operation and measurement circuitry of the quartz viscometer is described in detail. We provide evidence that the temperature of two-dimensional electron gas confined to a GaAs quantum well follows the temperature of the quartz viscometer down to 4 mK.

A new aromatic sulfonium counter-ion motif for polyoxometalate (POM) clusters with potential for structural and electronic fine-tuning has been designed. Its two derivatives 4-hydroxyphenyl dimethylsulfonium triflate (HPDST) and 4-(allyloxy)phenyl dimethylsulfonium triflate (APDST) exhibit ionic liquid behaviors under ambient conditions. HPDST and APDST are used to develop a series of aromatic sulfonium POM hybrids (HPDS/APDS)n[XMo12 O40] (HPDS and APDS are the cations of HPDST and APDST, respectively; X=P or Si; n=3 or 4), which are tested for photochromic behavior. On exposure to UV light, these POM hybrids undergo color change from yellow to green/blue. The coloration kinetics half lives (t1/2) are less for APDS-based hybrids than for HPDS-based hybrids, suggesting that alkyl substitution on the phenolic group helps to fine-tune the electron availability on the sulfonium moiety and hence to control the photochromic behavior of the POM hybrids. The t1/2 values of these hybrids are considerably lower than those of the reported aliphatic sulfonium POM hybrids. We have also demonstrated the application of photoreduced POM hybrids as catalysts for the reduction of 4-nitrophenol to 4-aminophenol.

In this work we report the opening of an energy gap at the filling factor ν = 3 + 1/3, firmly establishing the ground state as a fractional quantum Hall state. This and other odd-denominator states unexpectedly break particle-hole symmetry. Specifically, we find that the relative magnitudes of the energy gaps of the ν = 3 + 1/3 and 3 + 1/5 states from the upper spin branch are reversed when compared to the ν = 2 + 1/3 and 2 + 1/5 counterpart states in the lower spin branch. Our findings raise the possibility that the former states have a non-conventional origin.Over the last three decades we have wittnessed an ongoing exploration of topological phenomena in electronic systems. Topological ground states may arise from either single-particle band structure effects [1,2] or from emergent many-body effects in strongly interacting systems. One example of the latter is the fractional quantum Hall state (FQHS) at the Landau level filling factor ν = 1/3 [3], a ground state belonging to the larger class of conventional Laughlin-Jain FQHSs [4,5].More recently it was realized that the family of topological ground states may be much richer than previously thought. Of the novel FQHSs the ones supporting nonAbelian quasiparticles have generated the most excitement [6][7][8]. The FQHS at ν = 5/2 is believed to be such a non-Abelian state [9]. However, several other FQHSs in the region 2 < ν < 4, commonly called the second Landau level (SLL), are also thought to be non-Abelian [10][11][12][13][14][15].Despite sustained efforts in theory [10][11][12][13][14][15], the nature of the prominent odd-denominator FQHSs forming in the SLL,such as the ones at ν = 2 + 1/3 and 2 + 1/5, remains unknown. The FQHSs at ν = 2 + 1/3 [16-22] admits both non-Abelian candidate states [10,11] as well as a conventional Laughlin-Jain description [4,5]. The relatively poor overlap between the exact and numerically obtained wavefunctions [23][24][25][26][27][28] and the unusual excitations [15] does not provide firm evidence for Laughlin correlations in the ν = 2 + 1/3 FQHS. A number of recent experiments of the ν = 2 + 1/3 FQHS, however, found its bulk [21] and edge [29][30][31] properties consistent with a Laughlin description. The other prominent FQHS at ν = 2 + 1/5 [19,20] is generally believed to be of the conventional Laughlin type [25][26][27][28], although there is a non-Abelian construction for it as well [11]. It is therefore currently not clear whether or not the prominent odd-denominator FQHSs in the SLL, such as the ones at ν = 2 + 1/3 and 2 + 1/5, require a description beyond the conventional Laughlin-Jain theory.Experiments on the odd-denominator FQHS in the SLL have been restricted almost exclusively to the 2 < ν < 3 range, called the lower spin branch of the SLL (LSB SLL). Motivated by their poor understanding, we have performed transport studies of these FQHSs in the little known upper spin branch of the SLL (USB SLL), i.e. in the 3 < ν < 4 region. We establish a new FQHS at ν = 3+1/3 by detecting the opening of an energy gap. A quanti...