Trust in the Shadow of the Courts

If contract enforcers must be randomly selected from the same population and thus are as opportunistic as ordinary traders could a system of adjudication nevertheless increase the degree to which contractual obligations on large anonymous markets are fulfilled? Adopting an indirect evolutionary approach with endogenous preference formation it can be shown that without superior behaviour of adjudicators an adjudication system can induce untrustworthy traders to behave as if trustworthy. However, in the presence of occasional mistakes adjudication will merely slow down but not fully eliminate the evolutionary advantage of untrustworthy types. Only if arbitrators become judges who receive a fixed income occasional mistakes will not favour untrustworthy types. But even then under non-optimal court politics and unfavourable parameter constellations in a low trust environment the introduction of courts may in fact contribute to the crowding out of the trustworthy.

Time-resolved collapse and revival of the Kondo state near a quantum phase transition

One of the most successful paradigms of many-body physics is the concept of quasiparticles: excitations in strongly interacting matter behaving like weakly interacting particles in free space. Quasiparticles in metals are very robust objects. Yet, when a system's ground state undergoes a qualitative change at a quantum critical point (QCP), the quasiparticles may disintegrate and give way to an exotic quantum-fluid state of matter. The nature of this breakdown is intensely debated, because the emergent quantum fluid dominates the material properties up to high temperature and might even be related to the occurence of superconductivity in some compounds. Here we trace the dynamics of heavy-fermion quasiparticles in CeCu$_{6-x}$Au$_{x}$ and monitor their evolution towards the QCP in time-resolved experiments, supported by many-body calculations. A terahertz pulse disrupts the many-body heavy-fermion state. Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion state recovers, with a coherence time 100 times longer than typically associated with correlated metals. The quasiparticle weight collapses towards the QCP, yet its formation temperature remains constant -- phenomena believed to be mutually exclusive. Coexistence in the same experiment calls for revisions in our view on quantum criticality.Comment: Published version, including data on CeCu6, CeCu5.9Au0.1, and CeCu5Au1 and extended Supplementary Information. 7 pages, 4 figures, Supplementary Information: 5 pages, 3 figure

Cascade of magnetic field induced Lifshitz transitions in the ferromagnetic Kondo lattice material YbNi4P2

A ferromagnetic quantum critical point is thought not to exist in two and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi4(P,As)2, possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi4P2 experimentally but common probes such as ARPES and quantum oscillation measurements are lacking. Here, we studied the magnetic field dependence of transport and thermodynamic properties of YbNi4P2. The Kondo effect is continuously suppressed and additionally we identify nine Lifshitz transitions between 0.4 and 18 T. We analyze the transport coefficients in detail and identify the type of Lifshitz transitions as neck or void type to gain information on the Fermi surface of YbNi4P2. The large number of Lifshitz transitions observed within this small energy window is unprecedented and results from the particular flat renormalized band structure with strong 4f-electron character shaped by the Kondo lattice effect.Comment: 6 pages, 4 figure

Coupling between magnetic and thermodynamic properties in RRRh2_2Si2_2 (RR = Dy, Ho)

Single crystals of DyRh$_2$Si$_2$ and HoRh$_2$Si$_2$ were investigated by thermal expansion and magnetostriction. The different types of magnetic order can clearly be seen in these measurements, particularly the canting of the moments away from the crystallographic $c$ direction below about 12~K and the spin-flip for magnetic field applied along the $c$ direction. For HoRh$_2$Si$_2$, an additional transition just below $T_{\rm N}$ is analyzed by means of the Gr\"{u}neisen ratio and is likely caused by a change of the magnetic structure. Our results nicely corroborate findings from other magnetic and thermodynamic measurements on these materials and provide further evidence suggesting the formation of magnetic domains.Comment: 9 pages, 8 figure

Valence-transition-induced changes of the electronic structure in EuPd2_2Si2_2

We present results of hard X-ray angle-resolved photoemission spectroscopy and photoemission diffraction measurements performed on high-quality single crystals of the valence transition compound EuPd$_2$Si$_2$ for temperatures 25~K $\leq$ T $\leq$ 300~K. At low temperatures we observe a Eu $4f$ valence $v=2.5$, % occupation number $n=6.5$, which decreases to $v=2.1$ for temperatures above the valence transition around $T_V \approx 160$~K. The experimental valence numbers resulting from an evaluation of the Eu(III)/Eu(II) $3d$ core levels, are used for calculating band structures using density functional theory. The valence transition significantly changes the band structure as determined by angle-resolved photoemission spectroscopy. In particular, the Eu $5d$ valence bands are shifted to lower binding energies with increasing Eu $4f$ occupancy. To a lesser extent, bands derived from the Si $3p$ and Pd $4d$ orbitals are also affected. This observation suggests a partial charge transfer between Eu and Pd/Si sites. Comparison with {\it ab-initio} theory shows a good agreement with experiment, in particular concerning the unequal band shift with increasing Eu $4f$ occupancy

Study of J/ψJ/\psi and ψ(3686)→Σ(1385)0Σˉ(1385)0\psi(3686)\rightarrow\Sigma(1385)^{0}\bar\Sigma(1385)^{0} and Ξ0Ξˉ0\Xi^0\bar\Xi^{0}

We study the decays of $J/\psi$ and $\psi(3686)$ to the final states $\Sigma(1385)^{0}\bar\Sigma(1385)^{0}$ and $\Xi^0\bar\Xi^{0}$ based on a single baryon tag method using data samples of $(1310.6 \pm 7.0) \times 10^{6}$ $J/\psi$ and $(447.9 \pm 2.9) \times 10^{6}$ $\psi(3686)$ events collected with the BESIII detector at the BEPCII collider. The decays to $\Sigma(1385)^{0}\bar\Sigma(1385)^{0}$ are observed for the first time. The measured branching fractions of $J/\psi$ and $\psi(3686)\rightarrow\Xi^0\bar\Xi^{0}$ are in good agreement with, and much more precise, than the previously published results. The angular parameters for these decays are also measured for the first time. The measured angular decay parameter for $J/\psi\rightarrow\Sigma(1385)^{0}\bar\Sigma(1385)^{0}$, $\alpha =-0.64 \pm 0.03 \pm 0.10$, is found to be negative, different to the other decay processes in this measurement. In addition, the "12\% rule" and isospin symmetry in the $J/\psi$ and $\psi(3686)\rightarrow\Xi\bar\Xi$ and $\Sigma(1385)\bar{\Sigma}(1385)$ systems are tested.Comment: 11 pages, 7 figures. This version is consistent with paper published in Phys.Lett. B770 (2017) 217-22

Measurement of azimuthal asymmetries in inclusive charged dipion production in e+e−e^+e^- annihilations at s\sqrt{s} = 3.65 GeV

We present a measurement of the azimuthal asymmetries of two charged pions in the inclusive process $e^+e^-\rightarrow \pi\pi X$ based on a data set of 62 $\rm{pb}^{-1}$ at the center-of-mass energy $\sqrt{s}=3.65$ GeV collected with the BESIII detector. These asymmetries can be attributed to the Collins fragmentation function. We observe a nonzero asymmetry, which increases with increasing pion momentum. As our energy scale is close to that of the existing semi-inclusive deep inelastic scattering experimental data, the measured asymmetries are important inputs for the global analysis of extracting the quark transversity distribution inside the nucleon and are valuable to explore the energy evolution of the spin-dependent fragmentation function.Comment: 7 pages, 5 figure

Measurement of the e+e−→π+π−\mathrm e^+\mathrm e^-\rightarrow\mathrm\pi^+\mathrm\pi^- Cross Section between 600 and 900 MeV Using Initial State Radiation

We extract the $e^+e^-\rightarrow \pi^+\pi^-$ cross section in the energy range between 600 and 900 MeV, exploiting the method of initial state radiation. A data set with an integrated luminosity of 2.93 fb$^{-1}$ taken at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider is used. The cross section is measured with a systematic uncertainty of 0.9%. We extract the pion form factor $|F_\pi|^2$ as well as the contribution of the measured cross section to the leading order hadronic vacuum polarization contribution to $(g-2)_\mu$. We find this value to be $a_\mu^{\pi\pi,\rm LO}(600-900\;\rm MeV) = (368.2 \pm 2.5_{\rm stat} \pm 3.3_{\rm sys})\cdot 10^{-10}$.Comment: 14 pages, 7 figures, accepted by PL