Over the last decade, the interest in the spin-$1/2$ Heisenberg antiferromagnet (HAF) on the square kagome (also called shuriken) lattice has been growing as a model system of quantum magnetism with a quantum paramagnetic ground state, flat-band physics near the saturation field, and quantum scars. A further motivation to study this model comes from the recent discovery of a gapless spin liquid in the square kagome magnet $\mathrm{K}{\mathrm{Cu}}_{6}\mathrm{Al}\mathrm{Bi}{\mathrm{O}}_{4}{(\mathrm{S}{\mathrm{O}}_{4})}_{5}\mathrm{Cl}$ [M. Fujihala et al., Nat. Commun. 11, 3429 (2020)]. Here, we present large-scale numerical investigations of the specific heat $C(T)$, the entropy $S(T),$ as well as the susceptibility $\ensuremath{\chi}(T)$ by means of the finite-temperature Lanczos method for system sizes of $N=18,24,30,36,42,48$, and $N=54$. We find that the specific heat exhibits a low-temperature shoulder below the major maximum which can be attributed to low-lying singlet excitations filling the singlet-triplet gap, which is significantly larger than the singlet-singlet gap. This observation is further supported by the behavior of the entropy $S(T)$, where a change in curvature is present just at about $T/J=0.2$, the same temperature where the shoulder in $C$ sets in. For the susceptibility the low-lying singlet excitations are irrelevant, and the singlet-triplet gap leads to an exponentially activated low-temperature behavior. The maximum in $\ensuremath{\chi}(T)$ is found at a pretty low temperature ${T}_{\mathrm{max}}/J=0.146$ (for $N=42$) compared to ${T}_{\mathrm{max}}/J=0.935$ for the unfrustrated square-lattice HAF signaling the crucial role of frustration also for the susceptibility. We find a striking similarity of our square kagome data with the corresponding ones for the kagome HAF down to very low $T$. The magnetization process featuring plateaus and jumps and the field dependence of the specific heat that exhibits characteristic peculiarities attributed to the existence of a flat one-magnon band are discussed as well. 

/pdf/thermodynamics-of-the-spin-half-square-kagome-lattice-3nywy4am.pdf

Thermodynamics of the spin-half square kagome lattice antiferromagnet

At sufficiently low temperatures, interacting electron systems tend to develop orders. Exceptions are quantum critical point (QCP) and quantum spin liquid (QSL), where fluctuations prevent the highly entangled quantum matter to an ordered state down to the lowest temperature. While the ramification of these states may have appeared in high-temperature superconductors, ultra-cold atoms, frustrated magnets and quantum Moir\'e materials, their unbiased presence remain elusive in microscopic two-dimensional lattice models. Here, we show by means of large-scale quantum Monte Carlo simulations of correlated electrons on the $\pi$-flux square lattice subjected to extended Hubbard interaction, that a Gross-Neveu QCP separating massless Dirac fermions and an columnar valence bond solid at finite interaction, and a possible Dirac QSL at the infinite yet tractable interaction limit emerge in a coherent sequence. These unexpected novel quantum states resides in this simple-looking model, unifying ingredients including emergent symmetry, deconfined fractionalization and the dynamic coupling between emergent matter and gauge fields, will have profound implications both in quantum many-body theory and understanding of the aforementioned experimental systems. 

/pdf/dirac-fermions-with-plaquette-interactions-ii-su-4-phase-2wr1mdo3.pdf

Dirac fermions with plaquette interactions. II. SU(4) phase diagram with Gross-Neveu criticality and quantum spin liquid

The spin-1/2 kagom\'{e} Heisenberg antiferromagnet is generally accepted as one of the most promising two-dimensional models to realize a quantum spin liquid state. Previous experimental efforts were almost exclusively on only one archetypal material, the herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$, which unfortunately suffers from the notorious orphan spins problem caused by magnetic disorders. Here we turn to YCu$_3$(OH)$_{6.5}$Br$_{2.5}$, recently recognized as another host of a globally undistorted kagom\'{e} Cu$^{2+}$ lattice free from the orphan spins, thus a more feasible system for studying the intrinsic kagom\'{e} quantum spin liquid physics. Our high-resolution low-temperature thermal conductivity measurements yield a vanishing small residual linear term of $\kappa/T$ ($T\rightarrow 0$), and thus clearly rule out itinerant gapless fermionic excitations. Unusual scattering of phonons grows exponentially with temperature, suggesting thermally activated phonon-spin scattering and hence a gapped magnetic excitation, consistent with a $\mathbb{Z}_2$ quantum spin liquid ground state. Additionally, the analysis of magnetic field impact on the thermal conductivity reveals a field closing of the spin gap, while the excitations remain localized. 

/pdf/heat-transport-of-the-kagome-heisenberg-quantum-spin-liquid-2r5scotm.pdf

Heat transport of the kagome Heisenberg quantum spin liquid candidate 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>YCu</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>OH</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn

Abstract The search for the experimental evidence of quantum spin liquid (QSL) states is critical but extremely challenging, as the quenched interaction randomness introduced by structural imperfection is usually inevitable in real materials. YCu 3 (OH) 6.5 Br 2.5 (YCOB) is a spin-1/2 kagome Heisenberg antiferromagnet (KHA) with strong coupling of 〈 J 1 〉 ~ 51 K but without conventional magnetic freezing down to 50 mK ~ 0.001〈 J 1 〉. Here, we report a Br nuclear magnetic resonance (NMR) study of the local spin susceptibility and dynamics on the single crystal of YCOB. The temperature dependence of NMR main-line shifts and broadening can be well understood within the frame of the KHA model with randomly distributed hexagons of alternate exchanges, compatible with the formation of a randomness-induced QSL state at low temperatures. The in-plane spin fluctuations as measured by the spin-lattice relaxation rates (1/ T 1 ) exhibit a weak temperature dependence down to T ~ 0.03〈 J 1 〉. Our results demonstrate that the majority of spins remain highly fluctuating at low temperatures despite the quenched disorder in YCOB. 

/pdf/the-observation-of-quantum-fluctuations-in-a-kagome-3azyauaq.pdf

The observation of quantum fluctuations in a kagome Heisenberg antiferromagnet

In order to address the world-wide health challenge caused by the COVID-19 pandemic, the 3CL protease/SARS-CoV-2 main protease (SARS-CoV-2-Mpro) coded by its nsp5 gene became one of the biochemical targets...

On the origins of SARS-CoV-2 main protease inhibitors

We studied the magnetic properties of YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_{x}$] ($x$ = 0.33), where Cu$^{2+}$ ions form two-dimensional kagome layers. There is no magnetic order down to 50 mK while the Curie-Weiss temperature is on the order of -100 K. At zero magnetic field, the low-temperature specific heat shows a $T^2$ dependence. Above 2 T, a linear temperature dependence term in specific heat emerges, and the value of $\gamma = C/T$ increases linearly with the field. Furthermore, the magnetic susceptibility tends to a constant value at $T = 0$. Our results suggest that the magnetic ground state of YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_{x}$] is consistent with a Dirac quantum-spin-liquid state with a linearly dispersing spinon strongly coupled to an emergent gauge field, which has long been theoretically proposed as a candidate ground state in the two-dimensional kagome Heisenberg antiferromagnetic system. 

/pdf/possible-dirac-quantum-spin-liquid-in-the-kagome-quantum-1ttetw67.pdf

Possible Dirac quantum spin liquid in the kagome quantum antiferromagnet 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>YCu</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>OH</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mn>6</mml

Objective To evaluate the clinical efficacy and safety of leflunomide (L) added to the standard-of-care (SOC) treatment in COVID-19 patients hospitalised with moderate/critical clinical symptoms. Design Prospective, open-label, multicentre, stratified, randomised clinical trial. Setting Five hospitals in UK and India, from September 2020 to May 2021. Participants Adults with PCR confirmed COVID-19 infection with moderate/critical symptoms within 15 days of onset. Intervention Leflunomide 100 mg/day (3 days) followed by 10–20 mg/day (7 days) added to standard care. Primary outcomes The time to clinical improvement (TTCI) defined as two-point reduction on a clinical status scale or live discharge prior to 28 days; safety profile measured by the incidence of adverse events (AEs) within 28 days. Results Eligible patients (n=214; age 56.3±14.9 years; 33% female) were randomised to SOC+L (n=104) and SOC group (n=110), stratified according to their clinical risk profile. TTCI was 7 vs 8 days in SOC+L vs SOC group (HR 1.317; 95% CI 0.980 to 1.768; p=0.070). Incidence of serious AEs was similar between the groups and none was attributed to leflunomide. In sensitivity analyses, excluding 10 patients not fulfilling the inclusion criteria and 3 who withdrew consent before leflunomide treatment, TTCI was 7 vs 8 days (HR 1.416, 95% CI 1.041 to 1.935; p=0.028), indicating a trend in favour of the intervention group. All-cause mortality rate was similar between groups, 9/104 vs 10/110. Duration of oxygen dependence was shorter in the SOC+L group being a median 6 days (IQR 4–8) compared with 7 days (IQR 5–10) in SOC group (p=0.047). Conclusion Leflunomide, added to the SOC treatment for COVID-19, was safe and well tolerated but had no major impact on clinical outcomes. It may shorten the time of oxygen dependence by 1 day and thereby improve TTCI/hospital discharge in moderately affected COVID-19 patients. Trial registration numbers EudraCT Number: 2020-002952-18, NCT05007678.

/pdf/leflunomide-treatment-for-patients-hospitalised-with-covid-3vx3przn.pdf

Leflunomide treatment for patients hospitalised with COVID-19: DEFEAT-COVID randomised controlled trial

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Possible Dirac quantum spin liquid in the kagome quantum antiferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>YCu</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>OH</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mn>6</mml

Leflunomide treatment for patients hospitalised with COVID-19: DEFEAT-COVID randomised controlled trial