More Like this

1. Competing itinerant and local spin interactions in kagome metal FeGe

   https://doi.org/10.1038/s41467-023-44190-2  

   Chen, Lebing ; Teng, Xiaokun ; Tan, Hengxin ; Winn, Barry L. ; Granroth, Garrett E. ; Ye, Feng ; Yu, D. H. ; Mole, R. A. ; Gao, Bin ; Yan, Binghai ; et al ( March 2024 , Nature Communications)

   The combination of a geometrically frustrated lattice, and similar energy scales between degrees of freedom endows two-dimensional Kagome metals with a rich array of quantum phases and renders them ideal for studying strong electron correlations and band topology. The Kagome metal, FeGe is a noted example of this, exhibiting A-type collinear antiferromagnetic (AFM) order atT_N ≈ 400 K, then establishes a charge density wave (CDW) phase coupled with AFM ordered moment belowT_CDW ≈ 110 K, and finally forms ac-axis double cone AFM structure aroundT_Canting ≈ 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure of FeGe at temperatures well aboveT_CantingandT_CDWthat merge into gapped commensurate spin waves from the A-type AFM order. Commensurate spin waves follow the Bose factor and fit the Heisenberg Hamiltonian, while the incommensurate spin excitations, emerging belowT_Nwhere AFM order is commensurate, start to deviate from the Bose factor aroundT_CDW, and peaks atT_Canting. This is consistent with a critical scattering of a second order magnetic phase transition with decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order.

    

   more » « less
2. Anisotropic deconfined criticality in Dirac spin liquids

   https://doi.org/10.1007/JHEP07(2022)141  

   Shackleton, Henry ; Sachdev, Subir ( July 2022 , Journal of High Energy Physics)

   We analyze a Higgs transition from a U(1) Dirac spin liquid to a gapless ℤ₂spin liquid. This ℤ₂spin liquid is of relevance to the spinS= 1/2 square lattice antiferromagnet, where recent numerical studies have given evidence for such a phase existing in the regime of high frustration between nearest neighbor and next-nearest neighbor antiferromagnetic interactions (theJ₁-J₂model), appearing in a parameter regime between the vanishing of Néel order and the onset of valence bond solid ordering. The proximate Dirac spin liquid is unstable to monopole proliferation on the square lattice, ultimately leading to Néel or valence bond solid ordering. As such, we conjecture that this Higgs transition describes the critical theory separating the gapless ℤ₂spin liquid of theJ₁-J₂model from one of the two proximate ordered phases. The transition into the other ordered phase can be described in a unified manner via a transition into an unstable SU(2) spin liquid, which we have analyzed in prior work. By studying the deconfined critical theory separating the U(1) Dirac spin liquid from the gapless ℤ₂spin liquid in a 1/N_fexpansion, withN_fproportional to the number of fermions, we find a stable fixed point with an anisotropic spinon dispersion and a dynamical critical exponentz≠ 1. We analyze the consequences of this anisotropic dispersion by calculating the angular profiles of the equal-time Néel and valence bond solid correlation functions, and we find them to be distinct. We also note the influence of the anisotropy on the scaling dimension of monopoles.

    

   more » « less
3. Topological to magnetically ordered quantum phase transition in antiferromagnetic spin ladders with long-range interactions

   https://doi.org/10.21468/SciPostPhys.13.3.060  

   Yang, Luhang ; Weinberg, Phillip ; Feiguin, Adrian ( January 2022 , SciPost Physics)

   We study a generalized quantum spin ladder with staggered long rangeinteractions that decay as a power-law with exponent \alpha α .Using large scale quantum Monte Carlo (QMC) and density matrixrenormalization group (DMRG) simulations, we show that this modelundergoes a transition from a rung-dimer phase characterized by anon-local string order parameter, to a symmetry broken N'eel phase. Wefind evidence that the transition is second order. In the magneticallyordered phase, the spectrum exhibits gapless modes, while excitations inthe gapped phase are well described in terms of triplons – bound statesof spinons across the legs. We obtain the momentum resolved spin dynamicstructure factor numerically and find a well defined triplon band thatevolves into a gapless magnon dispersion across the transition. Wefurther discuss the possibility of deconfined criticality in thismodel. 

   more » « less
4. Fractional matter coupled to the emergent gauge field in a quantum spin ice

   Porée, Victor ; Yan, Han ; Desrochers, Félix ; Petit, Sylvain ; Lhotel, Elsa ; Appel, Markus ; Ollivier, Jacques ; Kim, Yong Baek ; Nevidomskyy, Andriy H. ; Sibille, Romain ( May 2023 , arXivorg)

   Electronic spins can form long-range entangled phases of condensed matter named quantum spin liquids. Their existence is conceptualized in models of two- or three-dimensional frustrated magnets that evade symmetry-breaking order down to zero temperature. Quantum spin ice (QSI) is a theoretically well-established example described by an emergent quantum electrodynamics, with excitations behaving like photon and matter quasiparticles. The latter are fractionally charged and equivalent to the `spinons' emerging from coherent phases of singlets in one dimension, where clear experimental proofs of fractionalization exist. However, in frustrated magnets it remains difficult to establish consensual evidence for quantum spin liquid ground states and their fractional excitations. Here, we use backscattering neutron spectroscopy to achieve extremely high resolution of the time-dependent magnetic response of the candidate QSI material Ce2Sn2O7. We find a gapped spectrum featuring a threshold and peaks that match theories for pair production and propagation of fractional matter excitations (spinons) strongly coupled to a background gauge field. The observed peaks provide evidence for a QSI through spectroscopic signatures of space-time symmetry fractionalization, while the threshold behavior corroborates the regime of strong light-matter interaction predicted for the emergent universe in a QSI. 

   more » « less
5. Fractional and composite excitations of antiferromagnetic quantum spin trimer chains

   https://doi.org/10.1038/s41535-021-00416-4  

   Cheng, Jun-Qing ; Li, Jun ; Xiong, Zijian ; Wu, Han-Qing ; Sandvik, Anders W. ; Yao, Dao-Xin ( January 2022 , npj Quantum Materials)

   Using quantum Monte Carlo, exact diagonalization, and perturbation theory, we study the spectrum of theS = 1/2 antiferromagnetic Heisenberg trimer chain by varying the ratiog = J₂/J₁of the intertrimer and intratrimer coupling strengths. The doublet ground states of trimers form effective interactingS = 1/2 degrees of freedom described by a Heisenberg chain. Therefore, the conventional two-spinon continuum of width ∝ J₁wheng = 1 evolves into to a similar continuum of width ∝ J₂wheng → 0. The intermediate-energy and high-energy modes are termeddoublonsandquartonswhich fractionalize with increasinggto form the conventional spinon continuum. In particular, atg ≈ 0.716, the gap between the low-energy spinon branch and the high-energy band with mixed doublons, quartons, and spinons closes. These features should be observable in inelastic neutron scattering experiments if a quasi-one-dimensional quantum magnet with the linear trimer structure andJ₂ < J₁can be identified. Our results may open a window for exploring the high-energy fractional excitations.

    

   more » « less