19 research outputs found
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 NambuGoldstone modes whereas the
massive amplitude mode, analogous to the Higgs boson in particle physics, is
prone to decay into a pair of low-energy NambuGoldstone 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 CHNCuBr 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
CHNCuBr. 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 Nel ordered state
breaks down at and above a critical pressure 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
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 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