qoi2dmagneticsusceptibility.hh

#ifndef QOI2DMAGNETICSUSCEPTIBILITY_HH
#define QOI2DMAGNETICSUSCEPTIBILITY_HH QOI2DMAGNETICSUSCEPTIBILITY_HH
#include "action/qft/nonlinearsigmaaction.hh"
#include "common/samplestate.hh"
#include "lattice/lattice2d.hh"
#include "mpi/mpi_wrapper.hh"
#include "qoi/quantityofinterest.hh"
#include <memory>

/** @file qoi2dmagneticsusceptibility.hh
 * @brief Header file for average squared magnetisation of non-linear O(3) sigma
 * model
 */

/** @class QoI2DMagneticSusceptibility
 *
 * @brief class for calculating the magnetic susceptibility of the non-linear
 * O(3) sigma model
 *
 * The returned magnetic susceptibility (average squared magnetisation)  is
 * defined as
 *
 * \f[
 *    \chi_t = 1/M*\langle \mu^2 \rangle
 * \f]
 *
 * where the 3-vector \f$\mu\f$ is the sum of all spins on the lattice and
 * \f$M\f$ is the number of lattice sites.
 *
 */

class QoI2DMagneticSusceptibility : public QoI {
public:
  /** @brief Create new instance
   *
   * @param[in] lattice_ Lattice
   */
  QoI2DMagneticSusceptibility(const std::shared_ptr<Lattice2D> lattice_)
      : lattice(lattice_) {}

  /** @brief Destructor */
  virtual ~QoI2DMagneticSusceptibility() {}

  /** @brief Evaluate on a state
   *
   * @param[in] phi_state State \f$\phi\f$ on which to evaluate the QoI
   */
  const double virtual evaluate(const std::shared_ptr<SampleState> phi_state);

private:
  /** @brief underlying lattice */
  const std::shared_ptr<Lattice2D> lattice;
};

/** @class QoI2DMagneticSusceptibilityFactory
 *
 * @brief Factory for constructing the QoI for a particular action
 */
class QoI2DMagneticSusceptibilityFactory : public QoIFactory {
public:
  /** @brief Return QoI for a specific  action
   *
   * @param[in] action Action to use
   */
  virtual std::shared_ptr<QoI> get(std::shared_ptr<Action> action) {
    std::shared_ptr<NonlinearSigmaAction> nonlinear_sigma_action;
    nonlinear_sigma_action =
        std::dynamic_pointer_cast<NonlinearSigmaAction>(action);
    std::shared_ptr<Lattice2D> lattice = nonlinear_sigma_action->get_lattice();
    return std::make_shared<QoI2DMagneticSusceptibility>(lattice);
  }
};

#endif // QOI2DMAGNETICSUSCEPTIBILITY_HH