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We investigate single crystals of the trigonal antiferromagnet EuZn$_2$P$_2$ ($P\overline{3}m1$) by means of electrical transport, magnetization measurements, X-ray magnetic scattering, optical reflectivity, angle-resolved photoemission spectroscopy (ARPES) and ab-initio band structure calculations (DFT+U). We find that the electrical resistivity of EuZn$_2$P$_2$ increases strongly upon cooling and can be suppressed in magnetic fields by several orders of magnitude (CMR effect). Resonant magnetic scattering reveals a magnetic ordering vector of $q = (0\, 0\, \frac{1}{2})$, corresponding to an $A$-type antiferromagnetic (AFM) order, below $T_{\rm N} = 23.7\,\rm K$. We find that the moments are canted out of the $a-a$ plane by an angle of about $40^{\circ}\pm 10^{\circ}$ degrees and tilted away from the [100] - direction by $30^{\circ}\pm 5^{\circ}$. We observe nearly isotropic magnetization behavior for low fields and low temperatures which is consistent with the magnetic scattering results. The magnetization measurements show a deviation from the Curie-Weiss behavior below $\approx 150\,\rm K$, the temperature below which also the field dependence of the material's resistivity starts to increase. An analysis of the infrared reflectivity spectrum at $T=295\,\rm K$ allows us to resolve the main phonon bands and intra-/interband transitions, and estimate indirect and direct band gaps of $E_i^{\mathrm{opt}}=0.09\,\rm{eV}$ and $E_d^{\mathrm{opt}}=0.33\,\rm{eV}$, respectively, which are in good agreement with the theoretically predicted ones. The experimental band structure obtained by ARPES is nearly $T$-independent above and below $T_{\rm N}$. The comparison of the theoretical and experimental data shows a weak intermixing of the Eu 4$f$ states close to the $\Gamma$ point with the bands formed by the phosphorous 3$p$ orbitals leading to an induction of a small magnetic moment at the P sites.

Under temperature or pressure tuning, tetragonal EuPd$_2$Si$_2$ is known to undergo a valence transition from nearly divalent to nearly trivalent Eu accompanied by a volume reduction. Albeit intensive work, its microscopic origin is still being discussed. Here, we investigate the mechanism of the valence transition under volume compression by $ab~initio$ density functional theory (DFT) calculations. Our analysis of the electronic and magnetic properties of EuPd$_2$Si$_2$ when approaching the valence transition shows an enhanced $c$-$f$ hybridization between localized Eu 4$f$ states and itinerant conduction states (Eu 5$d$, Pd 4$d$, and Si 3$p$) where an electronic charge redistribution takes place. We observe that the change in the electronic structure is intimately related to the volume reduction where Eu-Pd(Si) bond lengths shorten and, for the transition to happen, we trace the delicate balance between electronic bandwidth, crystal field splitting, Coulomb repulsion, Hund's coupling and spin-orbit coupling. In a next step we compare and benchmark our DFT results to surface-sensitive photoemission data in which the mixed-valent properties of EuPd$_2$Si$_2$ are reflected in a simultaneous observation of divalent and trivalent signals from the Eu $4f$ shell. The study serves as well to explore the limits of density functional theory and the choice of exchange correlation functionals to describe such a phenomenon as a valence transition., Comment: 8 pages, 5 figures, Accepted for publication in Phys. Rev. B

We present results of magnetic susceptibility and thermal expansion measurements performed on high-quality single crystals of EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ for 0 $\leq$ x $\leq$ 0.2 and temperatures 2 K $\leq T \leq$ 300 K. Data were taken at ambient pressure and finite He-gas pressure $p$ $\leq$ 0.5 GPa. For x = 0 and ambient pressure we observe a pronounced valence-change crossover centred around $T'_V$ $\approx$ 160 K with a non-magnetic ground state. This valence-change crossover is characterized by an extraordinarily strong pressure dependence of d$T'_V$ /d$p$ = (80 $\pm 10)$ K/GPa. We observe a shift of $T'_V$ to lower temperatures with increasing Ge-concentration, reaching $T'_V$ $\approx$ 90 K for x = 0.1, while still showing a non-magnetic ground state. Remarkably, on further increasing x to 0.2 we find a stable Eu$^{(2+\delta)+}$ valence with long-range antiferromagnetic order below $T_N$ = (47.5 $\pm$ 0.1) K, reflecting a close competition between two energy scales in this system. In fact, by the application of hydrostatic pressure as small as 0.1 GPa, the ground state of this system can be changed from long-range antiferromagnetic order for $p$ $, Comment: 9 pages, 3 figures

A universal phenomenon in phase transitions is critical slowing down (CSD) - systems, after an initial perturbation, take an exceptionally long time to return to equilibrium. It is universally observed in the dynamics of bosonic excitations, like order-parameter collective modes, but it is not generally expected to occur for fermionic excitations because of the half-integer nature of the fermionic spin. Direct observation of CSD in fermionic excitations or quasiparticles would therefore be of fundamental significance. Here, we observe fermionic CSD in the heavy-fermion (HF) compound YbRh$_2$Si$_2$ by terahertz time-domain spectroscopy. HFs are compound objects with a strongly enhanced effective mass, composed of itinerant and localized electronic states. We see that near the quantum phase transition in YbRh$_2$Si$_2$ the build-up of spectral weight of the HFs towards the Kondo temperature $T_K\approx 25$ K is followed by a logarithmic rise of the quasiparticle excitation rate on the heavy-Fermi-liquid side of the quantum phase transition below $10$ K. A critical two-band HF liquid theory shows that this is indicative of fermionic CSD. This CSD is a clear indication that the HF quasiparticles experience a breakdown near the quantum phase transition, and the critical exponent of this breakdown introduces a classification of fermionic quantum phase transitions analogous to thermodynamic phase transitions - solution to a long-standing problem., Comment: 6 pages, 4 figures

We present a combined experimental and theoretical study of the two-dimensional electron states at the iridium-silicide surface of the antiferromagnet GdIr2Si2 above and below the Néel temperature. Using angle-resolved photoemission spectroscopy (ARPES) we find a significant spin-orbit splitting of the surface states in the paramagnetic phase. By means of ab initio density-functional-theory (DFT) calculations we establish that the surface electron states that reside in the projected band gap around the ¯¯¯¯M point exhibit very different spin structures which are governed by the conventional and the cubic Rashba effect. The latter is reflected in a triple spin winding, i.e., the surface electron spin reveals three complete rotations upon moving once around the constant energy contours. Below the Néel temperature, our ARPES measurements show an intricate photoemission intensity picture characteristic of a complex magnetic domain structure. The orientation of the domains, however, can be clarified from a comparative analysis of the ARPES data and their DFT modeling. To characterize a single magnetic domain picture, we resort to the calculations and scrutinize the interplay of the Rashba spin-orbit coupling field with the in-plane exchange field, provided by the ferromagnetically ordered 4f moments of the near-surface Gd layer., This work was supported by the German Research Foundation (DFG) through Grants No. KR3831/5-1, No. LA655/20-1, No. GRK1621, No. SFB1143 (Project No. 247310070). We acknowledge support from the Russian Foundation for Basic Research (Grant No. 20-32-70127) and Saint Petersburg State University (Grant No. ID 51126254). M.M.O. acknowledges the support by Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-103910GB-I00). A.Yu.V. acknowledges the support by RFBR, Project No. 19-32-90251. D.V.V. acknowledges financial support from the Spanish Ministry of Economy (MAT-2017-88374-P). We acknowledge MAX IV Laboratory for time on the Bloch Beamline under Proposal 20190824. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. The authors gratefully acknowledge the GWK support for funding this work by providing computing time through the Center for Information Services and HPC (ZIH) at TU Dresden.

The Beginners Lab Course in Physics is scheduled very early in the course of studies not only for students of physics but also for students for which physics is a minor subject. We provide all partipants with several auxilliary materials for the preparation of the experiments and for writing a protocol. In our study, we enlighten the effect of the usage of these materials on the success of passing the lab course. ----- Das Anf��ngerpraktikum Physik ist eine fr��h im Studienverlauf stattfindende Veranstaltung sowohl f��r Hauptfachstudierende Physik, alsauch f��r Studierende mit Nebenfach Physik. Die vorliegende Untersuchung beleuchtet den Effekt der Verwendung von Hilfsmitteln zur Versuchsvorbereitung und Protokollierung auf das erfolgreiche Bestehen des Praktikums., 11 pages in German

Single crystals of HoIr$_2$Si$_2$ with the body-centered ThCr$_2$Si$_2$-type structure ($I4/mmm$) were grown by Bridgman method from indium flux. Single crystal structure determination yielded a Si-z position of 0.378(1) in the structure. We excluded the presence of the high temperature phase with the primitive CaBe$_2$Ge$_2$-type structure ($P 4/n m m$) by powder X-ray diffraction. Magnetic measurements on the single crystals yield a Neel temperature of $T_{\rm N}=22\,\rm K$. In the inverse magnetic susceptibility a strong anisotropy with Weiss temperatures $\Theta_{W}^{001}=26\,\rm K$ and $\Theta_{W}^{100}=-26\,\rm K$ occurs above $T_{\rm N}$. The effective magnetic moment $\mu_{\rm eff}^{001}=10.64\mu_{B}$ and $\mu_{\rm eff}^{100}=10.53\mu_{B}$ is close to the expected value for a free Ho$^{3+}$ ion, $\mu_{\rm eff}^{calc}=10.6\mu_{B}$. The field dependent magnetization shows a step-like behaviour due to crystalline electric field effects.