[en] A multiparticle analog of the theorem that attractive potentials bind in one and two dimensions leads to stability of 'negative ions' and strict monotonicity of the ground state energy for neutral boson 'plasma' in the number of pairs

[en] We derive a determinant expression for overlaps of Bethe states of the XXZ spin chain with the Néel state, the ground state of the system in the antiferromagnetic Ising limit. Our formula, of determinant form, is valid for generic system size. Interestingly, it is remarkably similar to the well-known Gaudin formula for the norm of Bethe states, and to another recently-derived overlap formula appearing in the Lieb–Liniger model. (paper)

[en] We report on the quantum yield (η) and decay time (τ) measurements at room temperature for the bright red-orange (602 nm) luminescence from new germanium-vacancy (Ge-V) centers in nano- and microcrystalline diamonds synthesized at high pressure and high temperature. The values η = 3 ± 1% and τ = 6.2±0.2 ns were found. The Stokes shift measured as the energy difference between the maxima of the luminescence and luminescence excitation spectra is negligible. The relative intensity of the zero-phonon line constitutes up to 70% of the total intensity of the luminescence. Results of our ab initio DFT calculations for the ground-state electronic and vibrational structure of (Ge-V)^- in diamond are presented and discussed.

[en] The excitonic insulator (EI) is an intriguing phase of condensed excitons undergoing a BEC-type transition. A prominent candidate has been identified in Ta${}_2$NiSe${}_5$. Ultrafast spectroscopy allows tracing the coherent response of the EI condensate directly in the time domain. Probing the collective electronic response we can identify the Higgs-amplitude mode of the condensate. In addition we find a peculiar coupling of the EI phase to a low frequency phonon mode. We will discuss the transient response on multiple energies scales ranging from the exciton dynamics in the NIR down to the coherent THz response of the gap.

[en] The ground state energy E(R) is studied for -Δ+V(x) + W(R-x) when V and W are negative with compact support. In particular, in dimension 3, when -Δ+V and -Δ+W both have no bound states but both have zero energy resonances, it is proved that E(R) approximately equal to -βR^-2 for R large with β=.321651512..

No abstract available

[en] Ground state energies of Hamiltonians in nonrelativistic quantum mechanics are analyzed with respect to their second derivative as functions of a coupling constant. In one-dimensional systems a comparison of two Hamiltonians yields relative convexity of the ground state energies as implications of a relative convexity of the potentials, by way of the relative log-concavity of the ground state wave functions. (author)

[en] We have proposed an exactly solvable quantum spin-3/2 model on a square lattice. Its ground state is a quantum spin liquid with a half integer spin per unit cell. The fermionic excitations are gapless with a linear dispersion, while the topological 'vison' excitations are gapped. Moreover, the massless Dirac fermions are stable. Thus, this model is, to the best of our knowledge, the first exactly solvable model of half-integer spins whose ground state is an 'algebraic spin liquid.'

[en] Using numerical methods, we study the finite-field ground state of an S=1 zigzag ladder. Special attention is paid to the magnetization plateau which appears at a half of the saturation magnetization in the ground-state magnetization curve

[en] A review of the construction of ground states in string theory is given. Simple mathematical and physical aspects of the compactified dimensions such as tori and orbifolds are discussed