1,552 research outputs found
Tenfold way and many-body zero modes in the Sachdev-Ye-Kitaev model
The Sachdev-Ye-Kitaev (SYK) model, in its simplest form, describes k Majorana fermions with random all-to-all four-body interactions. We consider the SYK model in the framework of a many-body Altland-Zirnbauer classification that sees the system as belonging to one of eight (real) symmetry classes depending on the value of k mod 8. We show that, depending on the symmetry class, the system may support exact many-body zero modes with the symmetries also dictating whether these may have a nonzero contribution to Majorana fermions, i.e., single-particle weight. These zero modes appear in all but two of the symmetry classes. When present, they leave clear signatures in physical observables that go beyond the threefold (Wigner-Dyson) possibilities for level spacing statistics studied earlier. Signatures we discover include a zero-energy peak or hole in the single-particle spectral function, depending on whether symmetries allow or forbid zero modes to have single-particle weight. The zero modes are also shown to influence the many-body dynamics, where signatures include a nonzero long-time limit for the out-of-time-order correlation function. Furthermore, we show that the extension of the four-body SYK model by quadratic terms can be interpreted as realizing the remaining two complex symmetry classes; we thus demonstrate how the entire tenfold Altland-Zirnbauer classification may emerge in the SYK model
Symmetry classes, many-body zero modes, and supersymmetry in the complex Sachdev-Ye-Kitaev model
The complex Sachdev-Ye-Kitaev (cSYK) model is a charge-conserving model of
randomly interacting fermions. The interaction term can be chosen such that the
model exhibits chiral symmetry. Then, depending on the charge sector and the
number of interacting fermions, level spacing statistics suggests a fourfold
categorization of the model into the three Wigner-Dyson symmetry classes. In
this work, inspired by previous findings for the Majorana Sachdev-Ye-Kitaev
model, we embed the symmetry classes of the cSYK model in the Altland-Zirnbauer
framework and identify consequences of chiral symmetry originating from
correlations across different charge sectors. In particular, we show that for
an odd number of fermions, the model hosts exact many-body zero modes that can
be combined into a generalized fermion that does not affect the system's
energy. This fermion directly leads to quantum-mechanical supersymmetry that,
unlike explicitly supersymmetric cSYK constructions, does not require
fine-tuned couplings, but only chiral symmetry. Signatures of the generalized
fermion, and thus supersymmetry, include the long-time plateau in
time-dependent correlation functions of fermion-parity-odd observables: The
plateau may take nonzero value only for certain combinations of the fermion
structure of the observable and the system's symmetry class. We illustrate our
findings through exact diagonalization simulations of the system's dynamics.ERC Starting Grant No. 678795 TopInS
Master of Science
thesisPrevious correlational research suggests that familiarity with another's attitudes is beneficial for physical health and relationship outcomes. This study aimed to examine the causal effects of attitude familiarity on the stressfulness of interactions and individuals' perceptions of themselves, their partner, and the interaction. Participants were randomly assigned to an attitude familiarity, trait familiarity, or no familiarity condition and completed a discussion task with an assigned partner. Measures of cardiovascular reactivity and participant perception were obtained. In line with our predictions, systolic blood pressure reactivity was significantly lower during the discussion task in the familiarity conditions compared to the control condition. However, other significant main effects of condition on cardiovascular reactivity or self-report measures were found. Secondary analyses indicated that increases in attitude similarity and respect and liking for a partner's attitude or traits were associated with more favorable perceptions of the partner and interaction
On quantification of weak sequential completeness
We consider several quantities related to weak sequential completeness of a
Banach space and prove some of their properties in general and in -embedded
Banach spaces, improving in particular an inequality of G. Godefroy, N. Kalton
and D. Li. We show some examples witnessing natural limits of our positive
results, in particular, we construct a separable Banach space with the
Schur property that cannot be renormed to have a certain quantitative form of
weak sequential completeness, thus providing a partial answer to a question of
G. Godefroy.Comment: 9 page
Investigation of hopping transport in n a Si H c Si solar cells with pulsed electrically detected magnetic resonance
Hopping transport through heterostructure solar cells based on B doped crystalline silicon wafers with highly P doped hydrogenated amorphous silicon emitters with different thicknesses is investigated at T 10 K with pulsed electrically detected magnetic resonance. The measurements show that transport is dominated by conduction band tail states g amp; 8776; 2.0046 with a distribution of their mutual coupling strength. The signal intensity correlates to the sample thickness and the g factors do not exhibit an anisotropy which suggests that transport is still dominated by bulk properties of amorphous silicon. In addition, two broad Pdonor hyperfine satellites can be detected. Influences of interface defects such as Pb like states known from silicon dioxide interfaces are either suppressed by the high Fermi energy at the interface or not presen
Physical Vacuum Properties and Internal Space Dimension
The paper addresses matrix spaces, whose properties and dynamics are
determined by Dirac matrices in Riemannian spaces of different dimension and
signature. Among all Dirac matrix systems there are such ones, which nontrivial
scalar, vector or other tensors cannot be made up from. These Dirac matrix
systems are associated with the vacuum state of the matrix space. The simplest
vacuum system realization can be ensured using the orthonormal basis in the
internal matrix space. This vacuum system realization is not however unique.
The case of 7-dimensional Riemannian space of signature 7(-) is considered in
detail. In this case two basically different vacuum system realizations are
possible: (1) with using the orthonormal basis; (2) with using the
oblique-angled basis, whose base vectors coincide with the simple roots of
algebra E_{8}.
Considerations are presented, from which it follows that the least-dimension
space bearing on physics is the Riemannian 11-dimensional space of signature
1(-)& 10(+). The considerations consist in the condition of maximum vacuum
energy density and vacuum fluctuation energy density.Comment: 19 pages, 1figure. Submitted to General Relativity and Gravitatio
Site-selective measurement of coupled spin pairs in an organic semiconductor
From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet (S=0) and dark triplet quintet (S=1,2) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3â5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems
Elucidating the structural composition of a Fe-N-C catalyst by nuclear and electron resonance techniques
FeâNâC catalysts are very promising materials for fuel cells and metalâair batteries. This work gives fundamental insights into the structural composition of an FeâNâC catalyst and highlights the importance of an inâdepth characterization. By nuclearâ and electronâresonance techniques, we are able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of αâiron and, surprisingly, iron oxide. Our work makes it questionable to what extent FeN4 sites can be present in FeâNâC catalysts prepared by pyrolysis at 900â°C and above. The simulation of the iron partial density of phonon states enables the identification of three FeN4 species in our catalyst, one of them comprising a sixfold coordination with endâon bonded oxygen as one of the axial ligands
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