Colossal c-axis response and lack of rotational symmetry breaking within the kagome plane of the CsV3_3Sb5_5 superconductor

The kagome materials AV4$_3$Sb$_5$ (A = K, Rb, Cs) host an intriguing interplay between unconventional superconductivity and charge-density-waves. Here, we investigate CsV$_3$Sb$_5$ by combining high-resolution thermal-expansion, heat-capacity and electrical resistance under strain measurements. We directly unveil that the superconducting and charge-ordered states strongly compete, and that this competition is dramatically influenced by tuning the crystallographic c-axis. In addition, we report the absence of additional bulk phase transitions within the charge-ordered state, notably associated with rotational symmetry-breaking within the kagome planes. This suggests that any breaking of the C$_6$ invariance occurs via different stacking of C$_6$-symmetric kagome patterns. Finally, we find that the charge-density-wave phase exhibits an enhanced A$_{1g}$-symmetric elastoresistance coefficient, whose large increase at low temperature is driven by electronic degrees of freedom

Strain-Tuning of 2D and 3D Charge-Density Waves in High-Temperature Superconducting YBa2_{2}Cu3_{3}Oy_{\rm{y}}

Uniaxial pressure experiments in underdoped YBa$_{2}$Cu$_{3}$O$_{\rm{y}}$ provide an efficient approach to the control of the competition between charge-density waves (CDWs) and superconductivity. It can enhance the correlation volume of ubiquitous short-range CDW correlations and above a critical value, even induce a long-range CDW order otherwise only accessible through the suppression of superconductivity by large magnetic fields. Here we use x-ray diffraction with access to large areas of reciprocal space to study the evolution of long- and short-range CDWs with in-plane strains and as a function of doping. This further allows us to precisely monitor in-situ the structural changes induced by uniaxial pressurization of the crystals for a precise strain estimation in measurements up to $-0.85 \%$ compression. Interestingly, we uncover direct evidence for a competition between long- and short-range CDWs and show that the long-range CDW modulation remains incommensurate at all investigated strains and temperatures, showing neither signs of discommensurations nor a pair-density wave component at $\lambda_{\rm{PDW}} = 2\lambda_{\rm{CDW}}$ below $T_c$. We discuss the impact of structural disorder and the relationship of our findings to previous reports on nematicity in high-temperature superconducting cuprates. More generally, our results underscore the potential of strain tuning as a powerful tool for probing and manipulating competing orders in quantum materials.Comment: I. Vinograd and S. M. Souliou contributed equally to this wor

Structure and Dynamics of Magnetocaloric Materials

The search for more efficient use of energy has been leading to a growing interest in the research field of magnetocaloric materials. The magnetocaloric effect (MCE) describes the change of temperature or entropy of a material when exposed to a change of the magnetic field and forms the basis of magnetocaloric refrigeration technologies. This utilization of the effect can offer a novel method for cooling that is economically feasible and ecologically friendly, and hence the effect attracts the attention of many researches. MCE is identified by the temperature change ($\Delta$T$_{ad}$) in an adiabatic process, and by the entropy change ($\Delta$S$_{iso}$) in an isothermal process.Part of this thesis is devoted to the investigation of the magnetocaloric effect (MCE) by direct measurements in pulsed magnetic fields as well as by analyzing the magnetization and specific heat data collected in static magnetic fields. The emphasis is on the direct measurement of the adiabatic temperature change $\Delta$T$_{ad}$ in pulsed magnetic fields as it provides the opportunity to examine the sample-temperature response to the magnetic field on a time scale of about 10 to 100 ms, which is on the order of typical operation frequencies (10 - 100 Hz) of magnetocaloric cooling devices. Furthermore, the accessible magnetic field range is extended to beyond 70 T and the short pulse duration provides nearly adiabatic conditions during the measurement. In the last years there has been an upsurge in the knowledge of the MCE and many materials have been investigated for their MCE characteristics. In the context of this thesis, the magnetocaloric properties of the single crystalline compounds MnFe$_{4}$Si$_{3}$ and Mn$_{5}$Ge$_{3}$ are investigated. Moreover, the nuclear and magnetic structure of the AF1' phase of the single crystalline compound Mn$_{5}$Si$_{3}$ are determined. For the MnFe$_{4}$Si$_{3}$, we have studied the magnetic and magnetocaloric response to pulsed and static magnetic fields up to 50 T. We determine the adiabatic temperature change $\Delta$T$_{ad}$ directly in pulsed fields and compare to the results of magnetization and specific heat measurements in static magnetic fields. The high ability of cycling even in high fields confirms the high structural stability of MnFe$_{4}$Si$_{3}$ against field changes, an important property for applications. The magnetic response to magnetic fields up to $\mu_{0}$H = 35 T shows that the anisotropy can be overcome by fields of approx. 7 T. For the Mn$_{5}$Ge$_{3}$, we have investigated the field direction dependence of the thermo-magnetic behavior in single crystalline Mn$_{5}$Ge$_{3}$. The adiabatic temperature change $\Delta$T$_{ad}$ in pulsed fields, the isothermal entropy change $\Delta$S$_{iso}$ calculated from static magnetization measurements and the heat capacity have been determined for field parallel and perpendicular to the easy magnetic direction [001]. The isothermal magnetization measurements yield furthermore the uniaxial anisotropy constants in second and fourth order, K$_{1}$ and K$_{2}$. We discuss how the anisotropy affects the magneto-caloric effect (MCE) and compare the results to the related [...

Structure and Dynamics of Magnetocaloric Materials

The search for more efficient use of energy has been leading to a growing interest in the research field of magnetocaloric materials. The magnetocaloric effect (MCE) describes the change of temperature or entropy of a material when exposed to a change of the magnetic field and forms the basis of magnetocaloric refrigeration technologies [1].In the last years there has been an upsurge in the knowledge of the MCE and many materials have been investigated for their MCE characteristics [2]. In the context of this talk, I will present the field direction dependence of the thermo-magnetic behavior in single crystalline compounds MnFe4Si3 and Mn5Ge3. The emphasis will be on the direct measurement of the adiabatic temperature change ΔTad in pulsed magnetic fields as it provides the opportunity to examine the sample temperature response to the magnetic field on a time scale close to the real process used in applications [3]. A discussion of how the anisotropy affects the magnetocaloric effect and a comparison between MnFe4Si3 compound, which exhibits easy plane anisotropy, and Mn5Ge3 which features uniaxial anisotropy, will be also presented [4].The Mn5Si3 compound exhibits inverse MCE related to the antiferromagnetic order phase transition AF1 to AF2, and direct MCE related to the AF2 to the paramagnetic phase transitions. Previous studies indicate a transition from the AF1 to AF1' before reaching the AF2 phase [5]. The magnetic structures of the AF1 and AF2 phases have been established [6, 7], while the magnetic structure of the AF1' phase has not been studied before. Therefore, the second part of the talk will be devoted to discuss the results of the investigation of the nuclear and magnetic structure of the intermediate phase AF1' of the single crystalline compound Mn5Si3.[1] K. A. Gschneidner Jr. and V.K. Pecharsky, “Thirty years of near room temperature magnetic cooling: Where we are today and future prospects”, International Journal of Refrigeration, 31, 945(2008).[2] V. Franco, J. S. Blazquez, B. Ingale, and A. Conde, “The magnetocaloric effect and magnetic refrigeration near room temperature: Materials and models”, Annual Review of Materials Research, 42, 305(2012).[3] N. Maraytta, Y. Skourski, J. Voigt, K. Friese, M. G. Herrmann, J. Perßon, J. Wosnitza, S. M. Salman, and T. Brückel, “Direct measurements of the magneto-caloric effect of MnFe4Si3 in pulsed magnetic fields”, Journal of Alloys and Compounds, 805, 1161(2019).[4] N. Maraytta, J. Voigt, C. S. Mejia, K. Friese, Y. Skourski, J. Perßon, S. M. Salman, and T. Brückel, “Anisotropy of the magnetocaloric effect: Example of Mn5Ge3”, Journal of Applied Physics, 128, 103903(2020).[5] M. R. Silva, P. J. Brown, and J. B. Forsyth, “Magnetic moments and magnetic site susceptibilities in Mn5Si3”, Journal of Physics: Condensed Matter, 14, 8707(2002).[6] P. J. Brown, J. B. Forsyth, V. Nunez, and F. Tasset, “The low-temperature antiferromagnetic structure of Mn5Si3 revised in the light of neutron polarimetry”, Journal of Physics: Condensed Matter, 4, 10025 (1992).[7] P. J. Brown and J. B. Forsyth, “Antiferromagnetism in Mn5Si3: The magnetic structure of the AF2 phase at 70 K”, Journal of Physics: Condensed Matter, 7, 7619(1995)