Magnon diffuse scattering (MDS) signals could, in principle, be studied with high spatial resolution in scanning transmission electron microscopy (STEM), thanks to recent technological progress in electron energy-loss spectroscopy. However, detecting MDS signals in STEM is technically challenging due to their overlap with the much stronger thermal diffuse scattering (TDS) signals. In bcc Fe at 300 K, MDS signals greater than or comparable to TDS signals have been predicted to occur under the central Bragg disk, well into a currently inaccessible energy-loss region. Therefore, to successfully detect MDS in STEM, it is necessary to identify conditions in which TDS and MDS signals can be distinguished from one another in regions outside the central Bragg disk. Temperature may be a key factor due to the distinct thermal signatures of magnon and phonon signals. In this work, we present a study on the effects of temperature on MDS and TDS in bcc Fe—considering a detector outside the central Bragg disk and a fixed convergent electron probe—using the frozen phonon and frozen magnon multislice methods. Our study reveals that neglecting the effects of atomic vibrations causes the MDS signal to grow approximately linearly up to the Curie temperature of Fe, after which it exhibits less variation. The MDS signal displays an alternating behavior due to dynamical diffraction, instead of increasing monotonically as a function of thickness. The inclusion of the effects of atomic vibrations through a complex atomic electrostatic potential causes the linear growth of the MDS signal to change to a nonlinear behavior that exhibits a predominant peak for a sample of thickness 16.072 nm at 1100 K. In contrast, the TDS signal grows more linearly than the MDS signal through the studied temperature range but still exhibits appreciable dynamical diffraction effects. An analysis of the signal-to-noise ratio (SNR) shows that the MDS signal can be a statistically significant contribution to the total scattering intensity under realizable measurement conditions and feasible acquisition times. For example, our study found that a SNR of 3 can be achieved with a beam current of 1 nA in less than 30 min for the 16.072-nm-thick bcc Fe sample at 1100 K.

We explore vortex dynamics in a two-dimensional Josephson junction array of micron-size superconducting islands fabricated from an epitaxial Al/InAs superconductor-semiconductor heterostructure, with a global top gate controlling Josephson coupling and vortex pinning strength. With applied dc current, minima of differential resistance undergo a transition, becoming local maxima at integer and half-integer flux quanta per plaquette, $f$. The zero-field transition from the superconducting phase is split, but unsplit for the anomalous metal phase, suggesting that pinned vortices are absent or sparse in the superconducting phase, and abundant but frozen in the anomalous metal. The onset of the transition is symmetric around $f=1/2$ but skewed around $f=1$, consistent with a picture of dilute vortices/antivortices on top of a checkerboard $(f=1/2)$ or uniform array of vortices $(f=1)$. Transitions show good scaling but with exponents that differ from Mott values obtained earlier. Besides the skewing at $f=1$, transitions show an overall even-odd pattern of skewing around integer $f$ values, which we attribute to vortex commensuration in the square array leading to symmetries around half-integer $f$.

Monolayer (1L) transition-metal dichalcogenides (TMDCs) are of strong interest in nanophotonics due to their narrow-band intense excitonic transitions persisting up to room temperature. When brought into resonance with surface plasmon polariton (SPP) excitations of a conductive medium, opportunities arise for studying and engineering strong light-matter coupling. Here, we consider a very simple geometry, namely a planar stack composed of a thin silver film, an ${\mathrm{Al}}_2{\mathrm{O}}_3$ spacer, and a monolayer of $\mathrm{W}{\mathrm{S}}_2$. We perform total internal reflection ellipsometry, which combines spectroscopic ellipsometry with the Kretschmann-Raether-type surface plasmon resonance configuration. The combined amplitude and phase response of the reflected light at varied angles of incidence proves that despite the atomic thinness of $1\text{L-}\mathrm{W}{\mathrm{S}}_2$, the strong-coupling (SC) regime between $A$ excitons and SPPs propagating in the thin Ag film is reached. The phasor representation of $\rho =r_p/r_s$, where $r_p$ and $r_s$ are the Fresnel refection coefficients in $p$- and $s$-polarization, respectively, corroborates SC as $\rho$ undergoes a topology change indicated by the occurrence of a double point at the crossover from the weak- to the strong-coupling regime. Our findings are validated by both analytical transfer-matrix method calculations and numerical Maxwell simulations. The findings open up new perspectives for applications in plasmonic modulators and sensors benefitting from the tunability of the optical properties of $1\text{L-}\mathrm{TMDCs}$ by electric fields, electrostatic doping, light, and the chemical environment.

After a decade of many claims to the opposite, there now is a growing consensus that generic disordered quantum wires, e.g., the XXZ-Heisenberg chain, do not exhibit many-body localization (MBL)—at least not in a strict sense within a reasonable window of disorder values $W$. Specifically, computational studies of short wires exhibit an extremely slow but unmistakable flow of physical observables with increasing time and system size (“creep”) that is consistently directed away from (strict) localization. Our work sheds fresh light on delocalization physics: Strong sample-to-sample fluctuations indicate the absence of a generic time scale, i.e., of a naive “clock rate”; however, the concept of an “internal clock” survives, at least in an ensemble sense. Specifically, we investigate the relaxation of the imbalance $\mathcal{I}\left(t\right)$ and its temporal fluctuations $\mathcal{F}\left(t\right)$ and the entanglement and Rényi entropies, ${\mathcal{S}}_{\mathrm{e}}\left(t\right)$ and ${\mathcal{S}}_2\left(t\right)$, in a one-dimensional system of interacting disordered fermions. We observe that adopting ${\mathcal{S}}_{\mathrm{e}}\left(t\right),{\mathcal{S}}_2\left(t\right)$ as a measure for the internal time per sample reduces the sample-to-sample fluctuations but does not eliminate them. However, a (nearly) perfect collapse of the average $\overline{\mathcal{I}}\left(t\right)$ and $\overline{\mathcal{F}}\left(t\right)$ for different $W$ is obtained when plotted against ${\overline{\mathcal{S}}}_{\mathrm{e}}\left(t\right)$ or ${\overline{\mathcal{S}}}_2\left(t\right)$, indicating that the average entropy appropriately models the ensemble-averaged internal clock. We take the tendency for faster-than-logarithmic growth of ${\overline{\mathcal{S}}}_{\mathrm{e}}\left(t\right)$ together with the smooth dependency on $W$ of all our observables within the entire simulation window as support for the crossover scenario, discouraging an MBL transition within the traditional parametric-window of computational studies.

We build the quasiparticle picture for the tripartite mutual information (TMI) after quantum quenches in spin chains that can be mapped onto free-fermion theories. A nonzero TMI (equivalently, topological entropy) signals quantum correlations between three regions of a quantum many-body system. The TMI is sensitive to entangled multiplets of more than two quasiparticles, i.e., beyond the entangled-pair paradigm of the standard quasiparticle picture. Surprisingly, for some nontrivially entangled multiplets, the TMI is negative at intermediate times. This means that the mutual information is monogamous, similar to holographic theories. Oppositely, for multiplets that are “classically” entangled, the TMI is positive. Crucially, a negative TMI reflects that the entanglement content of the multiplets is not directly related to the Generalized Gibbs Ensemble (GGE) that describes the postquench steady state. Thus the TMI is the ideal lens to observe the weakening of the relationship between entanglement and thermodynamics. We benchmark our results in the $XX$ chain and in the transverse field Ising chain. In the hydrodynamic limit of long times and large intervals, with their ratio fixed, exact lattice results are in agreement with the quasiparticle picture.

We calculate the perpendicular electrical conductivity in twisted three-dimensional graphite (rotationally stacked graphite pieces) by using the effective continuum model and the recursive Green's function method. In the low twist angle regime $(\theta \lesssim 2^{\circ })$, the conductivity shows a nonmonotonic dependence with a peak and dip structure as a function of the twist angle. By analyzing the momentum-resolved conductance and the local density of states, this behavior is attributed to the Fano resonance between continuum states of bulk graphite and interface-localized states, which is a remnant of the flat band in the magic-angle twisted bilayer graphene. We also apply the formulation to the high-angle regime near the commensurate angle $\theta \approx 21.8^{\circ }$, and reproduce the conductance peak observed in the experiment.

Resonant x-ray diffraction (RXD) measurements at the Tb $L_3$ and the $M_5$ edges were carried out on a single crystal of a rare-earth tetraboride, ${\mathrm{TbB}}_4$, which exhibits antiferromagnetic (AFM) order breaking both space-inversion and time-reversal symmetries. Considering its crystallographic and magnetic symmetries, both the anisotropic tensor of susceptibility (ATS) scattering and the magnetic scattering contribute to space-group-forbidden odd00 reflections under the resonant conditions in the AFM phase. We found that the RXD intensity of odd00 reflections depends on the helicity of the circularly polarized incident x rays, which is reasonably explained in terms of the interference effect between the ATS and the magnetic scatterings. Furthermore, the circular dichroic RXD showed a sample-position dependence, which reflects spatial distributions of AFM domains in ${\mathrm{TbB}}_4$. Our study reveals that RXD using circularly polarized x rays is a powerful technique to probe not only the magnitude but also the sign of AFM order parameters and to spatially resolve AFM domains in materials breaking both space-inversion and time-reversal symmetries.

The topological classification of gapped band structures depends on the particular definition of topological equivalence. For translation-invariant systems, stable equivalence is defined by a lack of restrictions on the numbers of occupied and unoccupied bands, while imposing restrictions on one or both leads to “fragile” and “delicate” topology, respectively. In this article, we describe a homotopic classification of band structures—which captures the topology beyond the stable equivalence—in the presence of additional lattice symmetries. As examples, we present complete homotopic classifications for spinless band structures with twofold rotation, fourfold rotation and fourfold dihedral symmetries, both in presence and absence of time-reversal symmetry. Whereas the rules of delicate and fragile topology do not admit a bulk-boundary correspondence, we identify a version of stable topology, which restricts the representations of bands, but not their numbers, which does allow for anomalous states at symmetry-preserving boundaries, which are associated with nontrivial bulk topology.

Consistency between the exchange-correlation (XC) functional used during pseudopotential construction and planewave-based electronic structure calculations is important for an accurate and reliable description of the structure and properties of condensed-phase systems. In this work, we present a general scheme for constructing pseudopotentials with range-separated hybrid (RSH) XC functionals based on the solution of the all-electron radial integro-differential equation for a spherically symmetrized reference atomic configuration. As a proof of principle, we demonstrate pseudopotential construction with the Perdew-Burke-Ernzerhof (PBE), hybrid PBE (PBE0), Heyd-Scuseria-Ernzerhof RSH (HSE06), and screened RSH (SRSH, based on the long-range corrected LC-$\omega \mathrm{PBE0}$ RSH) XC functionals for a select set of atoms and then investigate the importance of pseudopotential consistency when computing band gaps, equilibrium lattice parameters, bulk moduli, and atomization energies of several solid-state systems. In doing so, we find that pseudopotential consistency errors tend to be systematic and can be as large as 0.1 eV (or $1.4\%$) when computing band gaps.

We perform a projective symmetry group classification of spin $S=1/2$ symmetric quantum spin liquids with different gauge groups on the diamond lattice. Employing the Abrikosov fermion representation, we obtain $8\mathrm{SU}\left(2\right), 62\mathrm{U}\left(1\right)$, and $80{\mathbb{Z}}_2$ algebraic PSGs. Constraining these solutions to mean-field parton Ansätze with short-range amplitudes, the classification reduces to only $2\mathrm{SU}\left(2\right), 7\mathrm{U}\left(1\right),$ and $8{\mathbb{Z}}_2$ distinctly realizable phases. We obtain both the singlet and triplet fields for all Ansätze, discuss the spinon dispersions, and present the dynamical spin structure factors within a self-consistent treatment of the Heisenberg Hamiltonian with up to third nearest neighbor couplings. Interestingly, we find that a zero-flux $\mathrm{SU}\left(2\right)$ state and some descendent $\mathrm{U}\left(1\right)$ and ${\mathbb{Z}}_2$ states host robust gapless nodal loops in their dispersion spectrum, owing their stability at the mean-field level to the projective implementation of rotoinversion and screw symmetries. A nontrivial connection is drawn between one of our $\mathrm{U}\left(1\right)$ spinon Hamiltonians (belonging to the nonprojective class) and the Fu-Kane-Mele model for a three-dimensional topological insulator on the diamond lattice. We show that Gutzwiller projection of the 0- and $\pi$-flux $\mathrm{SU}\left(2\right)$ spin liquids generates long-range Néel order.

A magnonic spin current crossing a ferromagnet-metal interface is accompanied by spin current shot noise arising from the discrete quanta of spin carried by magnons. In thin films, for example, the spin of so-called squeezed magnons has been shown to deviate from the common value $\hslash$, with corresponding changes in the spin noise. In experiments, spin currents are typically converted to charge currents via the inverse spin Hall effect. We here analyze the magnitude of the spin current shot noise in the charge channel for a typical electrically detected spin pumping experiment and find that the voltage noise originating from the spin current shot noise is much smaller than the inevitable Johnson-Nyquist noise. Furthermore, we find that due to the local nature of the spin-charge conversion, the ratio of spin current shot noise and Johnson-Nyquist noise cannot be systematically enhanced by tuning the sample geometry, in contrast to the linear increase in dc spin pumping voltage with sample length. Instead, the ratio depends sensitively on material-specific transport properties. Our analysis thus provides guidance for the experimental detection of squeezed magnons through spin pumping shot noise.

The static and dynamical properties of finite-size lattice quantum spin models which spontaneously break a continuous U(1) symmetry in the thermodynamic limit are of central importance for a wide variety of physical systems, from condensed matter to quantum simulation. Such systems are characterized by a Goldstone excitation branch, terminating in a zero mode whose theoretical treatment within a linearized approach leads to divergencies on finite-size systems, revealing that the assumption of symmetry breaking is ill defined away from the thermodynamic limit. In this work we show that, once all its nonlinearities are taken into account, the zero mode corresponds exactly to a U(1) quantum rotor, related to the Anderson tower of states expected in systems showing symmetry breaking in the thermodynamic limit. The finite-momentum modes, when weakly populated, can be instead safely linearized (namely treated within spin-wave theory) and effectively decoupled from the zero mode. This picture leads to an approximate separation of variables between rotor and spin-wave ones, which allows for a correct description of the ground-state and low-energy physics. Most importantly, it offers a quantitative treatment of the finite-size nonequilibrium dynamics—following a quantum quench—dominated by the zero mode, for which a linearized approach fails after a short time. Focusing on the $2d$ XX model with power-law decaying interactions, we compare our equilibrium predictions with unbiased quantum Monte Carlo results and exact diagonalization; and our nonequilibrium results with time-dependent variational Monte Carlo. The agreement is remarkable for all interaction ranges, and it improves the longer the range. Our rotor/spin-wave theory defines a successful strategy for the application of spin-wave theory and its extensions to finite-size systems at equilibrium or away from it.

The identification of microscopic systems describing the low-energy properties of correlated materials has been a central goal of spectroscopic measurements. We demonstrate how two-dimensional (2D) nonlinear spectroscopy can be used to distinguish effective spin systems whose linear responses show similar behavior. Motivated by recent experiments on the quasi-1D Ising magnet ${\mathrm{CoNb}}_2{\mathrm{O}}_6$, we focus on two proposed systems—the ferromagnetic twisted Kitaev spin chain with bond dependent interactions and the transverse field Ising chain. The dynamical spin structure factor probed in linear response displays similar broad spectra for both systems from their fermionic domain wall excitations. In sharp contrast, the 2D nonlinear spectra of the two systems show clear qualitative differences: those of the twisted Kitaev spin chain contain off-diagonal peaks originating from the bond dependent interactions and transitions between different fermion bands absent in the transverse field Ising chain. We discuss the different signatures of spin fractionalization in integrable and nonintegrable regimes of the systems and their connection to experiments.

The combined rotational and time-reversal symmetry breakings that define an altermagnet lead to an unusual $d$-wave (or $g$-wave) magnetization order parameter, which in turn can be modeled in terms of multipolar magnetic moments. Here, we show that such an altermagnetic order parameter couples to the dynamics of the lattice even in the absence of an external magnetic field. This coupling is analogous to the nondissipative Hall viscosity and describes the stress generated by a time-varying strain under broken time-reversal symmetry. We demonstrate that this effect generates a hybridized paramagnon-polaron mode, which allows one to assess altermagnetic excitations directly from the phonon spectrum. Using a scaling analysis, we also demonstrate that the dynamic strain coupling strongly affects the altermagnetic phase boundary, but in different ways in the thermal and quantum regimes. In the ground state for both 2D and 3D systems, we find that a hardening of the altermagnon mode leads to an extended altermagnetic ordered regime, whereas for nonzero temperatures in 2D, the softening of the phonon modes leads to increased fluctuations that lower the altermagnetic transition temperature. In 3D even at finite temperatures, the dominant effect is the suppression of quantum fluctuations. We also discuss the application of these results to standard ferromagnetic systems.

We analyze the polarization of nuclear spins in a quantum dot induced by a single-electron spin that is electrically driven to perform coherent Rabi oscillations. We derive the associated nuclear-spin polarization rate and analyze its dependence on the accessible control parameters, especially the detuning of the driving frequency from the electron Larmor frequency. The arising nuclear-spin polarization is related to the Hartmann-Hahn effect known from the NMR literature with two important differences. First, in quantum dots, one typically uses a micromagnet, leading to a small deflection of the quantization axes of the electron and nuclear spins. Second, the electric driving wiggles the electron with respect to the atomic lattice. The two effects, absent in the traditional Hartmann-Hahn scenario, give rise to two mechanisms of nuclear-spin polarization in gated quantum dots. The arising nuclear-spin polarization is a resonance phenomenon, achieving maximal efficiency at the resonance of the electron Rabi and nuclear Larmor frequency (typically a few or a few tens of MHz). As a function of the driving frequency, the polarization rate can develop sharp peaks and reach large values at them. Since the nuclear polarization is experimentally detected as changes of the electron Larmor frequency, we often convert the former to the latter in our formulas and figures. In these units, the polarization can reach hundreds of MHz/s in GaAs quantum dots and at least tens of kHz/s in Si quantum dots. We analyze possibilities to exploit the resonant polarization effects for achieving large nuclear polarization and for stabilizing the Overhauser field through feedback.

We present a systematic topological theory of the scattering matrix and its submatrices, focusing on the singular values and vectors. We study the topological properties of the scattering matrix in the parameter space and determine a set of topological characteristics for a general system, including the winding number, Berry phase, and skew polarization. We reveal unique topological effects for a reciprocal system such as quantized single-band polarization and ${\mathbb{Z}}_2$ topology of Takagi vectors. Applying our theory, we reveal the topological nature of two phenomena: the well-known effect of coherent perfect absorption and a new effect we call coherent perfect extinction. Coherent perfect extinction refers to the complete extinction of light due to interference of multiple incident waves. It accounts for both absorption and scattering losses and encompasses many other known effects such as complete polarization conversion and reflectionless scattering modes, providing a unifying theme for these phenomena. Our analysis highlights the topological nature of these effects and provides criteria to localize their occurrence in parameter space, facilitating optical design to achieve these effects. We propose utilizing coherent perfect extinction for multichannel background-free sensing. These findings significantly advance our understanding of scattering phenomena and have important implications for the development of novel optical devices.

Scrambling of quantum information in unitary evolution can be hindered due to measurements and localization, which pin quantum mechanical wave functions in real space, suppressing entanglement in the steady state. In monitored free-fermionic models, the steady state undergoes an entanglement transition from a logarithmically entangled critical state to area law. However, disorder can lead to Anderson localization. We investigate free fermions in a random potential with continuous monitoring, which enables us to probe the interplay between measurement-induced and localized phases. We show that the critical phase is stable up to a finite disorder, and the criticality is consistent with the Berezinskii-Kosterlitz-Thouless universality. Furthermore, monitoring destroys localization, and the area-law phase at weak dissipation exhibits power-law decay of single-particle wave functions. Our work opens an avenue to probe this phase transition in electronic systems of quantum dot arrays and nanowires and allow quantum control of entangled states.

Recently, the bulk nickelate ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ is reported to show a signature of high-temperature superconductivity under high pressure above 14 GPa [H. Sun et al., Nature (London) 621, 493 (2023)]. We analyze the pairing mechanism and pairing symmetry in a bilayer Hubbard model with two orbitals in the $E_g$ multiplet. In the weak to moderate interaction regime, our functional renormalization group (FRG) calculations yield $s_{\pm }$-wave Cooper pairing triggered by leading spin fluctuations. The gap function changes sign across the Fermi pockets, and in real space the pairing is dominated by intra-unit-cell intraorbital components with antiphase between the on-site ones. In the strong-coupling limit, we develop a low-energy effective theory in terms of atomic one- and two-electron states in the $E_g$ multiplet. The variational treatment of the effective theory produces results consistent with the FRG ones, suggesting the robustness of such a pairing function. The driving force for superconductivity in the strong-coupling limit can be attributed to the local pair-hopping term and the spin exchange on vertical bonds. We also discuss a possible scenario for the weak insulating behavior under low pressures in terms of the tendency toward the formation of charge order in the strong-coupling limit.

The high-pressure behavior of monoclinic ${\mathrm{VO}}_2$ is revisited by a combination of Raman spectroscopy and x-ray diffraction on a single crystal under hydrostatic conditions at room temperature. A soft mode is observed up to $P_c=13.9\left(1\right)\mathrm{GPa}$. At this pressure, an isostructural phase transition between two monoclinic phases $M_1$ and $M_1^{\prime }$ hinders this instability. The features of this transformation (no apparent volume jump) indicate that the compression at ambient temperature passes close to a critical point. An analysis based on the Landau theory of phase transitions gives a complete description of the $P\text{−}T$ phase diagram. The $M_1^{\prime }$ is characterized by spontaneous displacements of the oxygen sublattice without any strong modification of the VV dimers distances nor the twist angle of vanadium chains. The spontaneous displacements of oxygen and the spontaneous deformations of the $\left(b_{M1},c_{M1}\right)$ plane follow the same quadratic dependence with pressure and scales with spontaneous shifts of the Raman phonons located at 225, 260, and $310\mathrm{c}{\mathrm{m}}^{-1}$. Pressure-induced shifts of the Raman peaks allow for new assignment of several Raman modes. In particular, the $A_g\left(1\right)+B_g\left(1\right)$ modes at $145\mathrm{c}{\mathrm{m}}^{-1}$ are identified as the vanadium displacive phonons. A second transformation in the metallic phase $X$, which is found triclinic ($P\overline{1}$) is observed starting at $32\mathrm{GPa}$, with a wide coexistence region (up to $42\mathrm{GPa}$). Upon decompression, phase $X$ transforms, between 20 and $3\mathrm{GPa}$, to another phase that is neither the $M_1^{\prime }$ nor $M_1$ phase. The structural transitions identified under pressure match with all the previously reported electronic modifications confirming that lattice and electronic degrees of freedom are closely coupled in this correlated material.

We report a detailed study of the specific heat and magnetic susceptibility of single crystals of a spin-liquid candidate: the hyperhoneycomb Cu oxalate framework compound $[{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3{\mathrm{NH}]}_2{\mathrm{Cu}}_2{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3$. The specific heat shows no anomaly associated with a magnetic transition at low temperatures down to $T\sim 180\mathrm{mK}$ in zero magnetic field. We observe a large linear-in-$T$ contribution to the specific heat $\gamma T, \gamma =98\left(1\right)\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$, at low temperatures, indicative of the presence of fermionic excitations despite the Mott insulating state. The low-$T$ specific heat is strongly suppressed by applied magnetic fields $H$, which induce an energy gap, $\mathrm{\Delta }\left(H\right)$, in the spin-excitation spectrum. We use the four-component relativistic density-functional theory (DFT) to calculate the magnetic interactions, including the Dzyaloshinskii-Moriya antisymmetric exchange, which causes an effective staggered field acting on one copper sublattice. The magnitude and field dependence of the field-induced gap, $\mathrm{\Delta }\left(H\right)\propto H^{2/3}$, are accurately predicted by the soliton mass calculated from the sine-Gordon model of weakly coupled antiferromagnetic Heisenberg chains with all parameters determined by our DFT calculations. Thus our experiment and calculations are entirely consistent with a model of $[{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3{\mathrm{NH}]}_2{\mathrm{Cu}}_2{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3$ in which anisotropic magnetic exchange interactions due to Jahn-Teller distortion cause one copper sublattice to dimerize, leaving a second sublattice of weakly coupled antiferromagnetic chains. We also show that this model quantitatively accounts for the measured temperature-dependent magnetic susceptibility. Thus $[{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3{\mathrm{NH}]}_2{\mathrm{Cu}}_2{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3$ is a canonical example of a one-dimensional spin-1/2 Heisenberg antiferromagnet and not a resonating-valence-bond quantum spin liquid, as previously proposed.