Exactly solvable spin- 1/2 XYZ models with highly degenerate partially ordered ground states

Exactly solvable models play a special role in condensed matter physics, serving as secure theoretical starting points for investigation of new phenomena. Changlani et al. [Phys. Rev. Lett. 120, 117202 (2018)] have discovered a limit of the XXZ model for S=1/2 spins on the kagome lattice, which is not only exactly solvable, but features a huge degeneracy of exact ground states corresponding to solutions of a three-coloring problem. This special point of the model was proposed as a parent for multiple phases in the wider phase diagram, including quantum spin liquids. Here, we show that the construction of Changlani et al. can be extended to more general forms of anisotropic exchange interaction, finding a line of parameter space in an XYZ model which maintains both the macroscopic degeneracy and the three-coloring structure of solutions. We show that the ground states along this line are partially ordered, in the sense that infinite-range correlations of some spin components coexist with a macroscopic number of undetermined degrees of freedom. We therefore propose the exactly solvable limit of the XYZ model on corner-sharing triangle-based lattices as a tractable starting point for discovery of quantum spin systems which mix ordered and spin-liquid-like properties

Superconductivity due to fluctuating loop currents

Orbital magnetism and the loop currents (LC) that accompany it have been proposed to emerge in many systems, including cuprates, iridates, and kagome superconductors. In the case of cuprates, LCs have been put forward as the driving force behind the pseudogap, strange-metal behavior, and $d_{x^2-y^2}$-wave superconductivity. Here, we investigate whether fluctuating intra-unit-cell loop currents can cause unconventional superconductivity. For odd-parity LCs, we find that they are strongly repulsive in all pairing channels near the underlying quantum-critical point (QCP). For even-parity LCs, their fluctuations do give rise to unconventional pairing. However, this pairing is not amplified in the vicinity of the QCP, in sharp contrast to other known cases of pairing mediated by intra-unit-cell order parameters, such as spin-magnetic, nematic, or ferroelectric ones. Applying our formalism to the cuprates, we conclude that pairing mediated by fluctuating intra-unit-cell LCs is unlikely to yield $d_{x^2-y^2}$-wave superconductivity. We also show that loop currents, if relevant for the cuprates, must vary between unit cells and break translation symmetry.Comment: 12 pages, 7 figure

Constraints on the superconducting state of Sr2RuO4 from elastocaloric measurements

Funding: This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – TRR 288-422213477 Elasto-Q-Mat project A05 (R.V.), project A07 (G.P. and J.S.), and project A10 (C.H. and A.P.M.). A.R. acknowledges support from the Engineering and Physical Sciences Research Council (Grant No. EP/P024564/1, EP/S005005/1, and EP/V049410/1). Research in Dresden benefits from the environment provided by the DFG Cluster of Excellence ct.qmat (EXC 2147, project ID 390858940).Strontium ruthenate Sr2RuO4 is an unconventional superconductor whose pairing symmetry has not been fully clarified, despite more than two decades of intensive research. Recent NMR Knight shift experiments have rekindled the Sr2RuO4 pairing debate by giving strong evidence against all odd-parity pairing states, including chiral p-wave pairing that was for a long time the leading pairing candidate. Here, we exclude additional pairing states by analyzing recent elastocaloric measurements [Y.S. Li et al., Nature 607, 276 (2022)]. To be able to explain the elastocaloric experiment, we find that unconventional even-parity pairings must include either large dx2−y2 -wave or large {dxz | dyz}-wave admixtures, where the latter possibility arises because of the body-centered point group symmetry. These {dxz | dyz}-wave admixtures take the form of distinctively body-centered-periodic harmonics that have horizontal line nodes. Hence gxy(x2−y2 )-wave and dxy-wave pairings are excluded as possible dominant even pairing states.PostprintPeer reviewe

T_c and the elastocaloric effect of Sr₂RuO₄ under ⟨110⟩ uniaxial stress:No indications of transition splitting

There is considerable evidence that the superconductivity of Sr2RuO4 has two components. Among this evidence is a jump in the shear elastic modulus c66 at the critical temperature Tc, observed in ultrasound measurements. Such a jump is forbidden for homogeneous single-component order parameters, and it implies that Tc should develop as a cusp under the application of shear strain with ⟨110⟩ principal axes. This shear strain should split the onset temperatures of the two components, if they coexist, or select one component if they do not. Here, we report measurements of Tc and the elastocaloric effect of Sr2RuO4 under uniaxial stress applied along the [110] lattice direction. Within experimental resolution, we resolve neither a cusp in the stress dependence of Tc, nor any second transition in the elastocaloric effect data. We show that reconciling these null results with the observed jumps in c66 requires extraordinarily fine tuning to a triple point of the Ginzburg-Landau parameter space. In addition, our results are inconsistent with homogeneous time-reversal symmetry breaking at a temperature T2 ≤ Tc as identified in muon spin relaxation experiments

Physical limitations of the Hohenberg-Mermin-Wagner argument

Hohenberg–Mermin–Wagnerov (HMW) teorem egzaktan je rezultat koji zabranjuje spontano narušenje neprekidnih simetrija za brojne modele u jednoj i dvjema dimenzijama. U ovome radu istražili smo fizikalna ograničenja ovoga teorema. U poglavlju 1 temeljito razmatramo HMW argument i uloge svih njegovih sastavnih dijelova. U istome poglavlju istražujemo osjetljivost HMW argumenta na konačnost sustava i smetnje koje narušavaju simetrije te ustvrđujemo da je argument u dvjema dimenzijama iznimno osjetljiv na oba učinka, dok u jednoj dimenziji nije. Energetska skala koja se prirodno pojavljuje u HMW argumentu određuje koliko je izražena ta osjetljivost. Zatim, u poglavlju 2, dokazujemo jedan vrlo općenit HMW teorem za supravodiču te istražujemo njegovu osjetljivost i mogućnosti daljnjeg poopćenija. Za sustave koji nisu astronomskih veličina, ustanovili smo da je u dvije dimenzije HMW argument, ali i infracrvene fluktuacije općenitije, previše slab da bi potisnuo supravodljivo uređenje. Rad završavamo novim argumentom koji daje netrivijalnu gornju među na temperaturu supravodljivog prijelaza.The Hohenberg–Mermin–Wagner (HMW) theorem is an exact result which rules out the spontaneous breaking of continuous symmetries for a variety of model systems in one and two dimensions. In this thesis, we explore the physical limitations of this theorem. An in-depth discussion of the HMW argument and how all of its parts fit together is given in Chapter 1. In the same chapter, we explore the sensitivity of the HMW argument to finite-size effects and symmetry-breaking perturbations and establish that in two dimensions it is exceptionally sensitive to both, whereas in one dimension it is not. The energy scale that naturally appears in the HMW argument determines how acute this sensitivity is. Afterwards, in Chapter 2 we prove a very general HMW theorem for superconductors and explore its sensitivity and possible further generalizations. For systems of non-astronomical size, we find that in two dimensions the HMW argument, and infrared fluctuations more generally, are too weak to suppress superconducting order. We conclude this work with a novel argument that gives a nontrivial upper bound on the superconducting transition temperature

Physical limitations of the Hohenberg-Mermin-Wagner argument

Hohenberg–Mermin–Wagnerov (HMW) teorem egzaktan je rezultat koji zabranjuje spontano narušenje neprekidnih simetrija za brojne modele u jednoj i dvjema dimenzijama. U ovome radu istražili smo fizikalna ograničenja ovoga teorema. U poglavlju 1 temeljito razmatramo HMW argument i uloge svih njegovih sastavnih dijelova. U istome poglavlju istražujemo osjetljivost HMW argumenta na konačnost sustava i smetnje koje narušavaju simetrije te ustvrđujemo da je argument u dvjema dimenzijama iznimno osjetljiv na oba učinka, dok u jednoj dimenziji nije. Energetska skala koja se prirodno pojavljuje u HMW argumentu određuje koliko je izražena ta osjetljivost. Zatim, u poglavlju 2, dokazujemo jedan vrlo općenit HMW teorem za supravodiču te istražujemo njegovu osjetljivost i mogućnosti daljnjeg poopćenija. Za sustave koji nisu astronomskih veličina, ustanovili smo da je u dvije dimenzije HMW argument, ali i infracrvene fluktuacije općenitije, previše slab da bi potisnuo supravodljivo uređenje. Rad završavamo novim argumentom koji daje netrivijalnu gornju među na temperaturu supravodljivog prijelaza.The Hohenberg–Mermin–Wagner (HMW) theorem is an exact result which rules out the spontaneous breaking of continuous symmetries for a variety of model systems in one and two dimensions. In this thesis, we explore the physical limitations of this theorem. An in-depth discussion of the HMW argument and how all of its parts fit together is given in Chapter 1. In the same chapter, we explore the sensitivity of the HMW argument to finite-size effects and symmetry-breaking perturbations and establish that in two dimensions it is exceptionally sensitive to both, whereas in one dimension it is not. The energy scale that naturally appears in the HMW argument determines how acute this sensitivity is. Afterwards, in Chapter 2 we prove a very general HMW theorem for superconductors and explore its sensitivity and possible further generalizations. For systems of non-astronomical size, we find that in two dimensions the HMW argument, and infrared fluctuations more generally, are too weak to suppress superconducting order. We conclude this work with a novel argument that gives a nontrivial upper bound on the superconducting transition temperature

Physical limitations of the Hohenberg-Mermin-Wagner argument

Hohenberg–Mermin–Wagnerov (HMW) teorem egzaktan je rezultat koji zabranjuje spontano narušenje neprekidnih simetrija za brojne modele u jednoj i dvjema dimenzijama. U ovome radu istražili smo fizikalna ograničenja ovoga teorema. U poglavlju 1 temeljito razmatramo HMW argument i uloge svih njegovih sastavnih dijelova. U istome poglavlju istražujemo osjetljivost HMW argumenta na konačnost sustava i smetnje koje narušavaju simetrije te ustvrđujemo da je argument u dvjema dimenzijama iznimno osjetljiv na oba učinka, dok u jednoj dimenziji nije. Energetska skala koja se prirodno pojavljuje u HMW argumentu određuje koliko je izražena ta osjetljivost. Zatim, u poglavlju 2, dokazujemo jedan vrlo općenit HMW teorem za supravodiču te istražujemo njegovu osjetljivost i mogućnosti daljnjeg poopćenija. Za sustave koji nisu astronomskih veličina, ustanovili smo da je u dvije dimenzije HMW argument, ali i infracrvene fluktuacije općenitije, previše slab da bi potisnuo supravodljivo uređenje. Rad završavamo novim argumentom koji daje netrivijalnu gornju među na temperaturu supravodljivog prijelaza.The Hohenberg–Mermin–Wagner (HMW) theorem is an exact result which rules out the spontaneous breaking of continuous symmetries for a variety of model systems in one and two dimensions. In this thesis, we explore the physical limitations of this theorem. An in-depth discussion of the HMW argument and how all of its parts fit together is given in Chapter 1. In the same chapter, we explore the sensitivity of the HMW argument to finite-size effects and symmetry-breaking perturbations and establish that in two dimensions it is exceptionally sensitive to both, whereas in one dimension it is not. The energy scale that naturally appears in the HMW argument determines how acute this sensitivity is. Afterwards, in Chapter 2 we prove a very general HMW theorem for superconductors and explore its sensitivity and possible further generalizations. For systems of non-astronomical size, we find that in two dimensions the HMW argument, and infrared fluctuations more generally, are too weak to suppress superconducting order. We conclude this work with a novel argument that gives a nontrivial upper bound on the superconducting transition temperature

A walking roller chain

In this article, we study the motion of a vertically fixed rapidly spinning roller chain that, after being released, “walks” a certain distance along the floor. We construct a two-dimensional model of the roller chain. By numerically integrating its equations of motion, we make predictions on the distances travelled by the roller chain, the code of the simulation being available online in the supplements. Finally, we compare the predictions with experiments, discuss the results, and suggest topics for further research

Constraints on the superconducting state of Sr₂RuO₄ from elastocaloric measurements

Strontium ruthenate Sr2RuO4 is an unconventional superconductor whose pairing symmetry has not been fully clarified, despite more than two decades of intensive research. Recent NMR Knight shift experiments have rekindled the Sr2RuO4 pairing debate by giving strong evidence against all odd-parity pairing states, including chiral p-wave pairing that was for a long time the leading pairing candidate. Here, we exclude additional pairing states by analyzing recent elastocaloric measurements [Y.S. Li et al., Nature 607, 276 (2022)]. To be able to explain the elastocaloric experiment, we find that unconventional even-parity pairings must include either large dx2−y2 -wave or large {dxz | dyz}-wave admixtures, where the latter possibility arises because of the body-centered point group symmetry. These {dxz | dyz}-wave admixtures take the form of distinctively body-centered-periodic harmonics that have horizontal line nodes. Hence gxy(x2−y2 )-wave and dxy-wave pairings are excluded as possible dominant even pairing states

Constraints on the superconducting state of Sr2RuO4 from elastocaloric measurements

Strontium ruthenate Sr2RuO4 is an unconventional superconductor whose pairing symmetry has not been fully clarified, despite more than two decades of intensive research. Recent NMR Knight shift experiments have rekindled the Sr2RuO4 pairing debate by giving strong evidence against all odd-parity pairing states, including chiral p-wave pairing that was for a long time the leading pairing candidate. Here, we exclude additional pairing states by analyzing recent elastocaloric measurements [YS. Li et al., Nature 607, 276--280 (2022)]. To be able to explain the elastocaloric experiment, we find that unconventional even-parity pairings must include either large dx2−y2-wave or large {dxz∣dyz}-wave admixtures, where the latter possibility arises because of the body-centered point group symmetry. These {dxz∣dyz}-wave admixtures take the form of distinctively body-centered-periodic harmonics that have horizontal line nodes. Hence gxy(x2−y2)-wave and dxy-wave pairings are excluded as possible dominant even pairing states