Second harmonic AC calorimetry technique within a diamond anvil cell

Tuning the energy density of matter at high pressures gives rise to exotic and often unprecedented properties, e.g., structural transitions, insulator-metal transitions, valence fluctuations, topological order, and the emergence of superconductivity. The study of specific heat has long been used to characterize these kinds of transitions, but their application to the diamond anvil cell (DAC) environment has proved challenging. Limited work has been done on the measurement of specific heat within DACs, in part due to the difficult experimental setup. To this end we have developed a novel method for the measurement of specific heat within a DAC that is independent of the DAC design and therefore readily compatible with any DACs already performing high pressure resistance measurements. As a proof-of-concept, specific heat measurements of the MgB2 superconductor were performed, showing a clear anomaly at the transition temperature (Tc), indicative of bulk superconductivity. This technique allows for specific heat measurements at higher pressure than previously possible.Comment: 5 pages with 5 figure

Direct observation of the Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point

Spontaneous symmetry-breaking quantum phase transitions play an essential role in condensed matter physics. The collective excitations in the broken-symmetry phase near the quantum critical point can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless Nambu$-$Goldstone modes whereas the massive amplitude mode, analogous to the Higgs boson in particle physics, is prone to decay into a pair of low-energy Nambu$-$Goldstone modes in low dimensions. Especially, observation of a Higgs amplitude mode in two dimensions is an outstanding experimental challenge. Here, using the inelastic neutron scattering and applying the bond-operator theory, we directly and unambiguously identify the Higgs amplitude mode in a two-dimensional S=1/2 quantum antiferromagnet C$_9$H$_{18}$N$_2$CuBr$_4$ near a quantum critical point in two dimensions. Owing to an anisotropic energy gap, it kinematically prevents such decay and the Higgs amplitude mode acquires an infinite lifetime.Comment: 12 pages, 4 figures in the main text+3 figures in Supplementary Informatio

Phase diagram of the Shastry-Sutherland Compound SrCu2(BO3)2 under extreme combined conditions of field and pressure

Motivated by the intriguing properties of the Shastry-Sutherland compound SrCu2(BO3)2 under pressure, with a still debated intermediate plaquette phase appearing at around 20 kbar and a possible deconfined critical point at higher pressure upon entering the antiferromagnetic phase, we have investigated its high-field properties in this pressure range using tunnel diode oscillator (TDO) measurements. The two main new phases revealed by these measurements are fully consistent with those identified by infinite Projected Entangled Pair states (iPEPS) calculations of the Shastry-Sutherland model, a 1/5 plateau and a 10 x 2 supersolid. Remarkably, these phases are descendants of the full-plaquette phase, the prominent candidate for the intermediate phase of SrCu2(BO3)2. The emerging picture for SrCu2(BO3)2 is shown to be that of a system dominated by a tendency to an orthorhombic distortion at intermediate pressure, an important constraint on any realistic description of the transition into the antiferromagnetic phase

Towards understanding the magnetic properties of the breathing pyrochlore compound Ba3Yb2Zn5O11: A single crystal study

Ba3Yb2Zn5O11 is unique among breathing pyrochlore compounds for being in the nearly decoupled limit where inter-tetrahedron interactions are weak, hosting isolated clusters or "molecular magnet" like tetrahedra of magnetic ytterbium (Yb3+) ions. In this work, we present the first study carried out on single-crystal samples of the breathing pyrochlore Ba3Yb2Zn5O11, using a variety of magnetometry and neutron scattering techniques along with theoretical modeling. We employ inelastic neutron scattering to investigate the magnetic dynamics as a function of applied field (with respect to both magnitude and direction) down to a temperature of 70 mK, where inelastic scattering reveals dispersionless bands of excitations as found in earlier powder sample studies, in good agreement with a single-tetrahedron model. However, diffuse neutron scattering at zero field and dc-susceptibility at finite field exhibit features suggesting the presence of excitations at low-energy that are not captured by the single tetrahedron model. Analysis of the local structure down to 2 K via pair distribution function analysis finds no evidence of structural disorder. We conclude that effects beyond the single tetrahedron model are important in describing the low-energy, low temperature physics of Ba3Yb2Zn5O11, but their nature remains undetermined

Pressure-induced unconventional quantum phase transition with fractionalization in the coupled ladder antiferromagnet C9H18N2CuBr4

We present a comprehensive study of the effect of hydrostatic pressure on the magnetic structure and spin dynamics in the spin-1/2 coupled ladder compound C$_9$H$_{18}$N$_2$CuBr$_4$. The applied pressure is demonstrated as a parameter to effectively tune the exchange interactions in the spin Hamiltonian without inducing a structural transition. The single-crystal heat capacity and neutron diffraction measurements reveal that the N$\rm \acute{e}$el ordered state breaks down at and above a critical pressure $P_{\rm c}$$\sim$1.0 GPa through a continuous quantum phase transition. The thorough analysis of the critical exponents indicates that such transition with a large anomalous exponent $\eta$ into a quantum-disordered state cannot be described by the classic Landau's paradigm. Using inelastic neutron scattering and quantum Monte Carlo methods, the high-pressure regime is proposed as a $Z_2$ quantum spin liquid phase in terms of characteristic fully gapped vison-like and fractionalized excitations in distinct scattering channels.Comment: 6 pages and 4 figures in the main text. The Supplementary Material is available upon the reques