arXiv subjects

Heinz-Jürgen Schmidt

Publications and source records attributed to Heinz-Jürgen Schmidt.

At least 19 recordsLinked to original sources

arXiv2026

Generalized Bell inequalities and frustrated spin systems

We find a close correspondence between generalized Bell inequalities (GBI's) of a special kind and certain frustrated spin systems. For example, the Clauser-Horn-Shimony-Holt inequality corresponds to the frustrated square with the signs $+++-$ for the nearest neighbor interaction between the spins. Similarly, the Pearle-Braunstein-Cave inequality corresponds to a frustrated even ring with the corresponding signs $+\ldots +-$. Upon this correspondence, the violation of such inequalities by the entangled singlet state in quantum mechanics is equivalent to the spin system possessing a classical coplanar ground state, the energy of which is lower than the Ising ground state's energy. We propose a scheme which generates new inequalities and give further examples, the frustrated hexagon with additional diagonal bonds and the frustrated hypercubes in $n=3,4,5$ dimensions. Surprisingly, the hypercube in $n=4$ dimensions yields an inequality which is \emph{not} violated by the singlet state. We extend the correspondence to other entangled states and XXZ-models of spin systems.

cond-mat.mtrl-sci↗

arXiv2026

Quantum orientation, Noether structure, composition of systems and operations

In this paper we argue that, in addition to the statistical structure of quantum theory, another structure, referred to here as the ``Noether structure," is necessary to describe the composition of systems and to define completely positive operations. A Noether structure reflects the dual role of Hermitian operators as observables on the one hand and as generators of symmetry transformations on the other. This idea has been expressed in a similar form in the works of Alfsen and Shultz, who investigated the conditions under which the Jordan product can be extended to an associative product of operator algebras. Our investigations into the Noether structure and the composition of systems are limited to the finite-dimensional case and establish a connection to completely positive operations. In the case of pure operations, the latter can be characterized as orientation-preserving maps.

quant-ph↗

arXiv2026

A generalization of the Choi isomorphism with application to open quantum systems

Completely positive transformations play an important role in the description of state changes in quantum mechanics, including the time evolution of open quantum systems. One useful tool to describe them is the so-called Choi isomorphism, which maps completely positive transformations to positive semi-definite matrices. Accordingly, there are numerous proposals to generalize the Choi isomorphism. In the present paper, we show that the 1976 paper of Gorini, Kossakowski and Sudarshan (GKS) already holds the key for a further generalization and study the resulting GKS isomorphism. As an application, we compute the GKS matrix of the time evolution of a general open quantum system up to second order in time.

math-ph↗

arXiv2025

On the motion of a point charge in a plate capacitor considering influence effects

A point charge between the plates of a capacitor generates an influence charge distribution on the plates that modify the electric field acting upon the point charge. This effect is described by the well-known Dirichlet Green's function for the two parallel conducting plate problem for which we derive an infinite mirror charge series representation. At the line perpendicular to the plates and passing through the point charge this Green's function and hence the total force can be explicitly evaluated in terms of the psi function. For the motion of the point charge we develop an analytical approximation and compare it with the numerical integration of the exact equations of motion. The correction due to influence effects is shown to be of order $O(-λ\log λ)$ where $λ$ denotes the relative strength of the Green's function compared with the pure capacitor potential.

physics.class-ph↗

arXiv2025

Ground states of classical spin polygons: Rigorous results and examples

We present a comprehensive and rigorous analysis of the lowest energy configurations (LECs) of classical spin polygons characterized by arbitrary couplings between neighboring spin sites. Our study shows that these ground states exhibit either collinear or coplanar arrangements, which allows us to determine the precise boundaries between these two phases. By simultaneously applying a spin flip and a bond inversion, we simplify the LEC problem and reduce it to a specific scenario with predominantly ferromagnetic (FM) bonds and a single antiferromagnetic (AFM) bond. Hence, competing interactions are always present, but, nevertheless, in the well-defined ranges of the system parameters the collinear LEC is realized. The difference angles between neighboring spins within the LEC can be captured by a single Lagrange parameter. We analytically investigate its dependence on the AFM bond and arrive at revealing results. Similarly, we can analyze the energy of the LEC, which shows a pronounced maximum as a function of AFM bond. To illustrate our findings, we give various examples that clearly demonstrate these results.

cond-mat.stat-mech↗

arXiv2025

Coarse-grained entropy balance of the Szilard engine

In order to reconcile the entropy reduction of a system through external interventions that are linked to a measurement with the second law of thermodynamics, there are two main proposals: (i) The entropy reduction is compensated by the entropy increase as a result of the measurement on the system (``Szilard principle"). (ii) The entropy reduction is compensated by the entropy increase as a result of the erasure of the measurement results (``Landauer/Bennett principle"). It seems that the LB principle is widely accepted in the scientific debate. In contrast, in this paper we argue for a modified S principle and criticize the LB principle with regard to various points. Our approach is based on the concept of ``conditional action", which is developed in detail. To illustrate our theses, we consider the entropy balance of a variant of the well-known Szilard engine, understood as a classical mechanical system.

physics.class-ph↗

arXiv2025

Species of structure and physical dimensions

This study addresses the often underestimated importance of physical dimensions and units in the formal reconstruction of physical theories, focusing on structuralist approaches that use the concept of ``species of structure" as a meta-mathematical tool. Similar approaches also play a role in current philosophical debates on the metaphysical status of physical quantities. Our approach builds on an earlier proposal by Terence Tao. It involves the representation of fundamental physical quantities by one-dimensional real ordered vector spaces, while derived quantities are formulated using concepts from linear algebra, e.g. tensor products and dual spaces. As an introduction, the theory of Ohm's law is considered. We then formulate a reconstruction of the calculus of physical dimensions, including Buckingham's $Π$-theorem. Furthermore, an application of this method to the Newtonian theory of gravitating systems consisting of point particles is demonstrated, emphasizing the role of the automorphism group and its physical interpretations.

math-ph↗

arXiv2024

Galilean decoherence and quantum measurement

In this study, we present a modified quantum theory, denoted as $QT^\ast$, which introduces mass-dependent decoherence effects. These effects are derived by averaging the influence of a proposed global quantum fluctuation in position and velocity. While $QT^\ast$ is initially conceived as a conceptual framework - a ``toy theory" - to demonstrate the internal consistency of specific perspectives in the measurement process debate, it also exhibits physical features worthy of serious consideration. The introduced decoherence effects create a distinction between micro- and macrosystems, determined by a characteristic mass-dependent decoherence timescale, $τ(m)$. For macrosystems, $QT^\ast$ can be approximated by classical statistical mechanics (CSM), while for microsystems, the conventional quantum theory $QT$ remains applicable. The quantum measurement process is analyzed within the framework of $QT^\ast$, where Galilean decoherence enables the transition from entangled states to proper mixtures. This transition supports an ignorance-based interpretation of measurement outcomes, aligning with the ensemble interpretation of quantum states. To illustrate the theory's application, the Stern-Gerlach spin measurement is explored. This example demonstrates that internal consistency can be achieved despite the challenges of modeling interactions with macroscopic detectors.

quant-ph↗

arXiv2024

Noncoplanar orders and quantum disordered states in maple-leaf (anti)ferromagnets

A promising route towards the realization of chiral spin liquids is the quantum melting of classically noncoplanar spin states via quantum fluctuations. In the classical realm, such noncoplanar orders can effectively be stabilized by interactions beyond nearest neighbors. Motivated by the recent synthesis of materials with a maple-leaf lattice geometry, we study the effect of cross-plaquette couplings on elementary Heisenberg antiferromagnets for this geometry (as well as their ferromagnetic counterparts). We find a rich spectrum of noncoplanar states, including a novel icosahedral order as well as incommensurate spin spirals, using large-scale Monte Carlo simulations in combination with a semi-analytical analysis. To inspect the potential quantum melting of these states, we analyze the quantum $S = 1/2$ variant of these models using pseudo-fermion functional renormalization group (pf-FRG) simulations. Notably, we indeed find extended parameter regimes lacking long-range magnetic order -- in regions classically occupied by noncoplanar orders -- which we putatively identify with the possible formation of chiral quantum spin liquids.

cond-mat.str-el↗

arXiv2024

The magnetization process of classical Heisenberg magnets with non-coplanar cuboc ground states

We consider a classical Heisenberg model on the kagomé and the square kagomé lattice, where at zero magnetic field non-coplanar cuboctahedral ground states with twelve sublattices exist if suitable exchange couplings are introduced between the other neighbors. Such 'cuboc ground states' are remarkable because they allow for chiral ordering. For these models, we discuss the magnetization process in an applied magnetic field $H$ by both numerical and analytical methods. We find some universal properties that are present in all models. The magnetization curve $M(H)$ usually contains only non-linear components and there is at least one magnetic field driven phase transition. Details of the $M(H)$ curve such as the number and characteristics (continuous or discontinuous) of the phase transitions depend on the lattice and the details of the exchange between the further neighbors. Typical features of these magnetization processes can already be derived for a paradigmatic 12 spin model that we define in this work.

cond-mat.str-el↗

arXiv2024

Exotic magnetization curves in classical square-kagomé spin lattices

Classical spin systems with non-coplanar ground states typically exhibit nonlinear magnetization curves characterized by kinks and jumps. Our article briefly summarizes the most important related analytical results. In a comprehensive case study, we then address AF-square kagomé and AF/FM-square kagomé spin lattices equipped with additional cross-plaquette interactions. It is known that these systems have non-coplanar ground states that assume a cuboctahedral structure in the absence of a magnetic field. When a magnetic field $H$ is switched on, a rich variety of different phases develops from the cuboctahedral ground state, which are studied in their dependence on $H$ and a cross-plaquette coupling constant $J_3>0$. For the AF square-kagomé spin lattice, we carefully identify and describe seven phases that appear in a phase diagram with five triple points. The transitions between these phases are predominantly discontinuous, although two cases exhibit continuous transitions. In contrast, the phase diagram of the AF/FM square-kagomé model shows only four phases with a single triple point, but these also lead to exotic magnetization curves. Here, too, there are two types of phase boundaries belonging to continuous and discontinuous transitions.

cond-mat.str-el↗

arXiv2023

The magnetic field does not perform work -- or does it?

We investigate the question discussed in the literature as to whether the magnetic field can perform work using two models that describe interacting magnetic dipoles. In the first model, the dipoles are realized by rigidly rotating charge clouds, whereas in the second model, one of the two dipoles is described by a real macroscopic spin density. The theoretical foundations of the second model are formulated in a recently published paper. We obtain equations of motion and detailed energy balance equations for both cases, but the answer to the title question depends on the choice of criteria for ``magnetic work".

physics.class-ph↗

arXiv2023

Non-Coplanar Magnetic Orders in Classical Square-Kagome Antiferromagnets

Motivated by the recent synthesis of a number of Mott insulating square-kagome materials, we explore the rich phenomenology of frustrated magnetism induced by this lattice geometry, also referred to as the squagome or shuriken lattice. On the classical level, square-kagome antiferromagnets are found to exhibit extensive degeneracies, order-by-disorder, and non-coplanar ordering tendencies, which we discuss for an elementary, classical Heisenberg model with nearest-neighbor and cross-plaquette interactions. Having in mind that upon introducing quantum fluctuations non-coplanar order can melt into chiral quantum spin liquids, we provide detailed information on the multitude of non-coplanar orders, including some which break rotational symmetry (possibly leading to nematic quantum orders), as well as a number of (incommensurate) spin spiral phases. Using extensive numerical simulations, we also discuss the thermodynamic signatures of these phases, which often show multi-step thermal ordering. Our comprehensive discussion of the classical square-kagome Heisenberg model, often drawing comparisons to the conventional kagome antiferromagnet, sets the stage for future explorations of quantum analogs of the various phases, either conceptually such as in quantum spin-1/2 generalizations of our model or experimentally such as in the Cu-based candidate materials.

cond-mat.str-el↗

arXiv2023

Classical limit of Gibbs states for quantum spin systems

We study the relation between quantum mechanical and classical Gibbs states of spin systems with spin quantum number $s$. It is known that quantum states and observables can be represented by functions defined on the phase space ${\mathcal S}$, which in our case is the $N$-fold product of unit spheres. Therefore, the classical limit $s\to\infty$ of (suitably scaled) quantum Gibbs states can be described as the limit of functions defined on ${\mathcal S}$. We choose to approximate the exponential function of the Hamiltonian by a polynomial of degree $n$ and thus have to deal with the problem of the limit of double sequences (depending on $n$ and $s$) treated in the theorem of Moore-Osgood. The convergence of quantum Gibbs states to classical ones is illustrated by the example of the Heisenberg dimer. We apply our method to the explicit calculation of the phase space function describing spin monomials, and finally add some general remarks on the theory of spin coherent states.

math-ph↗

arXiv2022

Classical ground states of spin lattices

We present a generalization of the Luttinger-Tisza-Lyons-Kaplan (LTLK) theory of classical ground states of Bravais lattices with Heisenberg coupling to non-Bravais lattices. It consists of adding certain Lagrange parameters to the diagonal of the Fourier transformed coupling matrix analogous to the theory of the general ground state problem already published. This approach is illustrated by an application to a modified honeycomb lattice, which has exclusive three-dimensional ground states as well as a classical spin-liquid ground state for different values of the two coupling constants. Another example, the modified square lattice, shows that we can also obtain so-called incommensurable ground states by our method.

cond-mat.str-el↗

arXiv2022

Thermodynamics of the spin square

Small spin systems at the interface between analytical studies and experimental application have been intensively studied in recent decades. The spin ring consisting of four spins with uniform antiferromagnetic Heisenberg interaction is an example of a completely integrable system, in the double sense: quantum mechanical and classical. However, this does not automatically imply that the thermodynamic quantities of the classical system can also be calculated explicitly. In this work, we derive analytical expressions for the density of states, the partition function, specific heat, entropy, and susceptibility. These theoretical results are confirmed by numerical tests. This allows us to compare the quantum mechanical quantities for increasing spin quantum numbers $s$ with their classical counterparts in the classical limit $s\to \infty$. As expected, a good agreement is obtained, except for the low temperature region. However, this region shrinks with increasing $s$, so that the classical state variables emerge as envelopes of the quantum mechanical ones.

cond-mat.stat-mech↗

arXiv2022

Thermodynamics of the classical spin triangle

The classical spin system consisting of three spins with Heisenberg interaction is an example of a completely integrable mechanical system. In this paper we explicitly calculate thermodynamic quantities as density of states, specific heat, susceptibility and spin autocorrelation functions. These calculations are performed (semi-)analytically and shown to agree with corresponding Monte Carlo simulations. For the long-time autocorrelation function, we find, for certain values of the coupling constants, a decay to constant values in the form of an $1/t$ damped harmonic oscillation and propose a theoretical explanation.

cond-mat.stat-mech↗

arXiv2021

Einstein's photon box revisited

We present a reformulation of Bohr's analysis of Einstein's thought experiment with the photon box, which hopefully settles some unclear issues. The box is weighed by two position measurements, the first immediately before the emission of the photon and the second after a quarter period of oscillation of the box, or alternatively after a half period. These times are precisely defined with respect to the moving clock installed in the box, but vary statistically when measured with a stationary clock outside the box, due to gravitational time dilation in accordance with Bohr's original idea. Moreover, we will discuss two other proposals to solve the apparent inconsistency without invoking the theory of general relativity.

quant-ph↗