arXiv subjects

B. Dalla Piazza

Publications and source records attributed to B. Dalla Piazza.

13 recordsLinked to original sources

arXiv2025

Confined and deconfined spinon excitations in the rectangular-lattice quantum antiferromagnet

Fractionalization remains one of the most fascinating manifestations of strong interactions in quantum many-body systems. In quantum magnetism, the existence of spinons -- collective magnetic excitations that behave as quasiparticles with fractional quantum numbers -- is proven in spin chains, but the criteria for their appearance in higher dimensions remain disputed. Motivated by experiments reporting the observation of spinons at high energies in the square-lattice Heisenberg antiferromagnet, we adopt the approach of extrapolating from where spinons are well defined. We study the dynamical properties of a Gutzwiller-projected wave function, the staggered-flux state, on a rectangular spin-1/2 Heisenberg lattice as a function of the spatial coupling ratio, $γ= J_y/J_x$. By studying the spectrum and the spinon separation distribution we show how, as the system evolves from one-dimensional (1D) towards 2D, the spinons become progressively more confined over most of reciprocal space, but remain deconfined at specific wave vectors.

cond-mat.str-el↗

arXiv2020

Staggered-flux state for rectangular-lattice spin 1/2 Heisenberg antiferromagnet

We investigate the spin-1/2 Heisenberg model on a rectangular lattice, using the Gutzwiller projected variational wave function known as the staggered flux state. Using Monte Carlo techniques, the variational parameters and static spin-structure factor for different coupling anisotropies $γ=J_y/J_x$ are calculated. We observe a gradual evolution of the ground state energy towards a value which is very close to the 1D estimate provided by the Bethe ansatz and a good agreement between the finite size scaling of the energies. The spin-spin correlation functions exhibit a power-law decay with varying exponents for different anisotropies. Though the lack of Néel order makes the staggered flux state energetically unfavorable in the symmetric case $γ=1$, it appears to capture the essence of the system close to 1D. Hence we believe that the staggered flux state provides an interesting starting point to explore the crossover from quantum disordered chains to the Néel ordered 2D square lattices.

cond-mat.str-el↗

arXiv2016

Phase diagram of diluted Ising ferromagnet LiHo$_x$Y$_{1-x}$F$_4$

We present a systematic study of the phase diagram of LiHo$_x$Y$_{1-x}$F$_4$ ($0.25<x<1$) Ising ferromagnets obtained from neutron scattering measurements and mean-field calculations. We show that while the thermal phase transition decreases linearly with dilution, as predicted by mean-field theory, the critical transverse field at the quantum critical point is suppressed much faster. This behavior is related to competition between off-diagonal dipolar coupling and quantum fluctuations that are tuned by doping and applied field, respectively. In this paper, we quantify the deviation of the experimental results from mean-field predictions, with the aim that this analysis can be used in future theoretical efforts towards a quantitative description.

cond-mat.str-el↗

arXiv2015

Neutron spectroscopic study of crystal-field excitations and the effect of the crystal field on dipolar magnetism in Li$R$F$_4$ ($R$ = Gd, Ho, Er, Tm, and Yb)

We present a systematic study of the crystal field interactions in the Li$R$F$_4$, $R$ = Gd, Ho, Er, Tm and Yb, family of rare-earth magnets. Using detailed inelastic neutron scattering measurements we have been able to quantify the transition energies and wavefunctions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to lineshape broadening in LiTmF$_4$ and level splitting in LiYbF$_4$. Furthermore, using high resolution neutron scattering from LiHoF$_4$, we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.

cond-mat.str-el↗

arXiv2015

Spin-Orbit-Induced Orbital Excitations in Sr2RuO4 and Ca2RuO4: A Resonant Inelastic X-ray Scattering Study

High-resolution resonant inelastic X-ray scattering (RIXS) at the oxygen K-edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism X-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p-orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λ_{so}=200~meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.

cond-mat.str-el↗

arXiv2015

Fractional excitations in the square-lattice quantum antiferromagnet

The square-lattice quantum Heisenberg antiferromagnet displays a pronounced anomaly of unknown origin in its magnetic excitation spectrum. The anomaly manifests itself only for short wavelength excitations propagating along the direction connecting nearest neighbors. Using polarized neutron spectroscopy, we have fully characterized the magnetic fluctuations in the model metal-organic compound CFTD, revealing an isotropic continuum at the anomaly indicative of fractional excitations. A theoretical framework based on the Gutzwiller projection method is developed to explain the origin of the continuum at the anomaly. This indicates that the anomaly arises from deconfined fractional spin-1/2 quasiparticle pairs, the 2D analog of 1D spinons. Away from the anomaly the conventional spin-wave spectrum is recovered as pairs of fractional quasiparticles bind to form spin-1 magnons. Our results therefore establish the existence of fractional quasiparticles in the simplest model two dimensional antiferromagnet even in the absence of frustration.

cond-mat.str-el↗

arXiv2014

Non-equilibrium hysteresis and spin relaxation in the mixed-anisotropy dipolar coupled spin-glass LiHo$_{0.5}$Er$_{0.5}$F$_{4}$

We present a study of the model spin-glass LiHo$_{0.5}$Er$_{0.5}$F$_4$ using simultaneous AC susceptibility, magnetization and magnetocaloric effect measurements along with small angle neutron scattering (SANS) at sub-Kelvin temperatures. All measured bulk quantities reveal hysteretic behavior when the field is applied along the crystallographic c axis. Furthermore avalanche-like relaxation is observed in a static field after ramping from the zero-field-cooled state up to $200 - 300$ Oe. SANS measurements are employed to track the microscopic spin reconfiguration throughout both the hysteresis loop and the related relaxation. Comparing the SANS data to inhomogeneous mean-field calculations performed on a box of one million unit cells provides a real-space picture of the spin configuration. We discover that the avalanche is being driven by released Zeeman energy, which heats the sample and creates positive feedback, continuing the avalanche. The combination of SANS and mean-field simulations reveal that the conventional distribution of cluster sizes is replaced by one with a depletion of intermediate cluster sizes for much of the hysteresis loop.

cond-mat.str-el↗

arXiv2013

The electronic structure of the high-symmetry perovskite iridate Ba2IrO4

We report angle-resolved photoemission (ARPES) measurements, density functional and model tight-binding calculations on Ba$_2$IrO$_4$ (Ba-214), an antiferromagnetic ($T_N=230$ K) insulator. Ba-214 does not exhibit the rotational distortion of the IrO$_6$ octahedra that is present in its sister compound Sr$_2$IrO$_4$ (Sr-214), and is therefore an attractive reference material to study the electronic structure of layered iridates. We find that the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at the predominant role of the spin-orbit interaction in these materials. Temperature-dependent ARPES data show that the energy gap persists well above $T_N$, and favour a Mott over a Slater scenario for this compound.

cond-mat.str-el↗

arXiv2013

Phase diagram with enhanced spin-glass region of mixed Ising / XY magnets LiHo$_{x}$Er$_{1-x}$F$_{4}$

We present the experimental phase diagram of LiHo$_x$Er$_{1-x}$F$_4$, a dilution series of dipolar-coupled model magnets.The phase diagram was determined using a combination of AC susceptibility and neutron scattering. Three unique phases in addition to the Ising ferromagnet LiHoF$_4$ and the XY anti-ferromagnet LiErF$_4$ have been identified. Below $x=0.86$ an embedded spin-glass phase is observed, where a spin-glass exists within the ferromagnetic structure. Below $x=0.57$ an Ising spin-glass is observed consisting of frozen needle-like clusters. For $x \sim 0.3 - 0.1$ an antiferromagnetically coupled spin-glass occurs. A reduction of $T_C(x)$ for the ferromagnet is observed which disobeys the mean-field predictions that worked for LiHo$_x$Y$_{1-x}$F$_4$.

cond-mat.str-el↗

arXiv2012

Spin excitations in a single La$_2$CuO$_4$ layer

The dynamics of S=1/2 quantum spins on a 2D square lattice lie at the heart of the mystery of the cuprates \cite{Hayden2004,Vignolle2007,Li2010,LeTacon2011,Coldea2001,Headings2010,Braicovich2010}. In bulk cuprates such as \LCO{}, the presence of a weak interlayer coupling stabilizes 3D Néel order up to high temperatures. In a truly 2D system however, thermal spin fluctuations melt long range order at any finite temperature \cite{Mermin1966}. Further, quantum spin fluctuations transfer magnetic spectral weight out of a well-defined magnon excitation into a magnetic continuum, the nature of which remains controversial \cite{Sandvik2001,Ho2001,Christensen2007,Headings2010}. Here, we measure the spin response of \emph{isolated one-unit-cell thick layers} of \LCO{}. We show that coherent magnons persist even in a single layer of \LCO{} despite the loss of magnetic order, with no evidence for resonating valence bond (RVB)-like spin correlations \cite{Anderson1987,Hsu1990,Christensen2007}. Thus these excitations are well described by linear spin wave theory (LSWT). We also observe a high-energy magnetic continuum in the isotropic magnetic response. This high-energy continuum is not well described by 2 magnon LSWT, or indeed any existing theories.

cond-mat.str-el↗

arXiv2012

Magnetic hour-glass dispersion and its relation to high-temperature superconductivity in iron-tuned Fe$_{1+y}$Te$_{0.7}$Se$_{0.3}$

High-temperature superconductivity remains arguably the largest outstanding enigma of condensed matter physics. The discovery of iron-based high-temperature superconductors has renewed the importance of understanding superconductivity in materials susceptible to magnetic order and fluctuations. Intriguingly they show magnetic fluctuations reminiscent of the superconducting (SC) cuprates, including a 'resonance' and an 'hour-glass' shaped dispersion, which provide an opportunity to new insight to the coupling between spin fluctuations and superconductivity. Here we report inelastic neutron scattering data on Fe$_{1+y}$Te$_{0.7}$Se$_{0.3}$ using excess iron concentration to tune between a SC ($y=0.02$) and a non-SC ($y=0.05$) ground states. We find incommensurate spectra in both samples but discover that in the one that becomes SC, a constriction towards a commensurate hourglass shape develop well above $T_c$. Conversely a spin-gap and concomitant spectral weight shift happen below $T_c$. Our results imply that the hourglass shaped dispersion is most likely a pre-requisite for superconductivity, whereas the spin-gap and shift of spectral weight are consequences of superconductivity. We explain this observation by pointing out that an inwards dispersion towards the commensurate wave-vector is needed for the opening of a spin gap to lower the magnetic exchange energy and hence provide the necessary condensation energy for the SC state to emerge.

cond-mat.supr-con↗

arXiv2011

Unified quantitative model for magnetic and electronic spectra of the undoped cuprates

Using low-energy projection of the one-band t-t'-t"-Hubbard model we derive an effective spin-Hamiltonian and its spin-wave expansion to order 1/S. We fit the spin-wave dispersion of several parent compounds to the high-temperature superconducting cuprates: La2CuO4, Sr2CuO2Cl2 and Bi2Sr2YCu2O8. Our accurate quantitative determination of the one-band Hubbard model parameters allows prediction and comparison to experimental results of measurable quantities such as staggered moment, double occupancy density, spin-wave velocity and bimagnon excitation spectrum and density of states, which is discussed in relation to K-edge RIXS and Raman experiments.

cond-mat.str-el↗

arXiv2010

Measurement of magnetic excitations in the two-dimensional antiferromagnetic Sr2CuO2Cl2 insulator using resonant x-ray scattering:Evidence for extended interactions

Using high-resolution resonant inelastic x-ray scattering (RIXS), we performed a momentum-resolved study of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr_2CuCl_2O_2. We identify both a single-spin-wave feature and a multi-magnon continuum, and show that the X-ray polarization can be used to distinguish these two contributions in the cross-section. The spin-waves display a large (70 meV) dispersion between the zone-boundary points ($π$,0) and ($π$/2,$π$/2). Employing an extended $t$-$t'$-$t"$-$U$ one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions. We conclude that sizeable extended magnetic interactions are present in \scoc{} and probably important in all undoped cuprates.

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