60,662 research outputs found

    Resummation and the semiclassical theory of spectral statistics

    Full text link
    We address the question as to why, in the semiclassical limit, classically chaotic systems generically exhibit universal quantum spectral statistics coincident with those of Random Matrix Theory. To do so, we use a semiclassical resummation formalism that explicitly preserves the unitarity of the quantum time evolution by incorporating duality relations between short and long classical orbits. This allows us to obtain both the non-oscillatory and the oscillatory contributions to spectral correlation functions within a unified framework, thus overcoming a significant problem in previous approaches. In addition, our results extend beyond the universal regime to describe the system-specific approach to the semiclassical limit.Comment: 10 pages, no figure

    Towards Baxter equation in supersymmetric Yang-Mills theories

    Full text link
    We perform an explicit two-loop calculation of the dilatation operator acting on single trace Wilson operators built from holomorphic scalar fields and an arbitrary number of covariant derivatives in N=2 and N=4 supersymmetric Yang-Mills theories. We demonstrate that its eigenspectrum exhibits double degeneracy of opposite parity eigenstates which suggests that the two-loop dilatation operator is integrable. Moreover, the two-loop anomalous dimensions in the two theories differ from each other by an overall normalization factor indicating that the phenomenon is not sensitive to the presence of the conformal symmetry. Relying on these findings, we try to uncover integrable structures behind the two-loop dilatation operator using the method of the Baxter Q-operator. We propose a deformed Baxter equation which exactly encodes the spectrum of two-loop anomalous dimensions and argue that it correctly incorporates a peculiar feature of conformal scalar operators -- the conformal SL(2) spin of such operators is modified in higher loops by an amount proportional to their anomalous dimension. From the point of view of spin chains this property implies that the underlying integrable model is ``self-tuned'' -- the all-loop Hamiltonian of the spin chain depends on the total SL(2) spin which in its turn is proportional to the Hamiltonian.Comment: Latex, 18 pages, 3 figure

    Electronic phase separation due to magnetic polaron formation in the semimetallic ferromagnet EuB6_6 - A weakly-nonlinear-transport study

    Full text link
    We report measurements of weakly nonlinear electronic transport, as measured by third-harmonic voltage generation V3ωV_{3\omega}, in the low-carrier density semimetallic ferromagnet EuB6_6, which exhibits an unusual magnetic ordering with two consecutive transitions at Tc1=15.6T_{c_1} = 15.6\,K and Tc2=12.5T_{c_2} = 12.5\,K. Upon cooling in zero magnetic field through the ferromagnetic transition, the dramatic drop in the linear resistivity at the upper transition Tc1T_{c_1} coincides with the onset of nonlinearity, and upon further cooling is followed by a pronounced peak in V3ωV_{3 \omega} at the lower transition Tc2T_{c_2}. Likewise, in the paramagnetic regime, a drop of the material's magnetoresistance R(H)R(H) precedes a magnetic-field-induced peak in nonlinear transport. A striking observation is a linear temperature dependence of V3ωpeak(H)V_{3\omega}^{\rm peak}(H). We suggest a picture where at the upper transition Tc1T_{c_1} the coalescing MP form a conducting path giving rise to a strong decrease in the resistance. The MP formation sets in at around T35T^\ast \sim 35\,K below which these entities are isolated and strongly fluctuating, while growing in number. The MP then start to form links at Tc1T_{c_1}, where percolative electronic transport is observed. The MP merge and start forming a continuum at the threshold Tc2T_{c_2}. In the paramagnetic temperature regime Tc1<T<TT_{c_1} < T < T^\ast, MP percolation is induced by a magnetic field, and the threshold accompanied by charge carrier delocalization occurs at a single critical magnetization.Comment: to appear in J. Kor. Phys. Soc (ICM2012 conference contribution

    The measurement postulates of quantum mechanics are operationally redundant

    Get PDF
    Understanding the core content of quantum mechanics requires us to disentangle the hidden logical relationships between the postulates of this theory. Here we show that the mathematical structure of quantum measurements, the formula for assigning outcome probabilities (Born's rule) and the post-measurement state-update rule, can be deduced from the other quantum postulates, often referred to as "unitary quantum mechanics", and the assumption that ensembles on finite-dimensional Hilbert spaces are characterised by finitely many parameters. This is achieved by taking an operational approach to physical theories, and using the fact that the manner in which a physical system is partitioned into subsystems is a subjective choice of the observer, and hence should not affect the predictions of the theory. In contrast to other approaches, our result does not assume that measurements are related to operators or bases, it does not rely on the universality of quantum mechanics, and it is independent of the interpretation of probability.Comment: This is a post-peer-review, pre-copyedit version of an article published in Nature Communications. The final authenticated version is available online at: http://dx.doi.org/10.1038/s41467-019-09348-

    Thermodynamics of the frustrated J1J_1-J2J_2 Heisenberg ferromagnet on the body-centered cubic lattice with arbitrary spin

    Full text link
    We use the spin-rotation-invariant Green's function method as well as the high-temperature expansion to discuss the thermodynamic properties of the frustrated spin-SS J1J_{1}-J2J_{2} Heisenberg magnet on the body-centered cubic lattice. We consider ferromagnetic nearest-neighbor bonds J1<0J_1 < 0 and antiferromagnetic next-nearest-neighbor bonds J20J_2 \ge 0 and arbitrary spin SS. We find that the transition point J2cJ_2^c between the ferromagnetic ground state and the antiferromagnetic one is nearly independent of the spin SS, i.e., it is very close to the classical transition point J2c,clas=23J1J_2^{c,{\rm clas}}= \frac{2}{3}|J_1|. At finite temperatures we focus on the parameter regime J2<J2cJ_2<J_2^c with a ferromagnetic ground-state. We calculate the Curie temperature TC(S,J2)T_{C}(S,J_{2}) and derive an empirical formula describing the influence of the frustration parameter J2J_{2} and spin SS on TCT_C. We find that the Curie temperature monotonically decreases with increasing frustration J2J_2, where very close to J2c,clasJ_2^{c,{\rm clas}} the TC(J2)T_C(J_2)-curve exhibits a fast decay which is well described by a logarithmic term 1/log(23J1J2)1/\textrm{log}(\frac{2}{3}|J_1|-J_{2}). To characterize the magnetic ordering below and above TCT_C, we calculate the spin-spin correlation functions S0SR\langle {\bf S}_{\bf 0} {\bf S}_{\bf R} \rangle, the spontaneous magnetization, the uniform static susceptibility χ0\chi_0 as well as the correlation length ξ\xi. Moreover, we discuss the specific heat CVC_V and the temperature dependence of the excitation spectrum. As approaching the transition point J2cJ_2^c some unusual features were found, such as negative spin-spin correlations at temperatures above TCT_C even though the ground state is ferromagnetic or an increase of the spin stiffness with growing temperature.Comment: 19 pages, 10 figures, version as in EPJ
    corecore