scholarly journals Magnetically brightened dark electron-phonon bound states in a van der Waals antiferromagnet

2022 ◽  
Vol 13 (1) ◽  
Author(s):  
Emre Ergeçen ◽  
Batyr Ilyas ◽  
Dan Mao ◽  
Hoi Chun Po ◽  
Mehmet Burak Yilmaz ◽  
...  

AbstractIn van der Waals (vdW) materials, strong coupling between different degrees of freedom can hybridize elementary excitations into bound states with mixed character1–3. Correctly identifying the nature and composition of these bound states is key to understanding their ground state properties and excitation spectra4,5. Here, we use ultrafast spectroscopy to reveal bound states of d-orbitals and phonons in 2D vdW antiferromagnet NiPS3. These bound states manifest themselves through equally spaced phonon replicas in frequency domain. These states are optically dark above the Néel temperature and become accessible with magnetic order. By launching this phonon and spectrally tracking its amplitude, we establish the electronic origin of bound states as localized d–d excitations. Our data directly yield electron-phonon coupling strength which exceeds the highest known value in 2D systems6. These results demonstrate NiPS3 as a platform to study strong interactions between spins, orbitals and lattice, and open pathways to coherent control of 2D magnets.

2019 ◽  
Author(s):  
Ελένη Αζά

The discovery of materials with coexisting magnetic and ferroelectric orders, has revived theinterest of condensed matter physics and materials’ science communities maintaining the greatpromise of such fundamental mechanisms in devising applications ranging from portablemagnetoelectric (ME) sensors and memories to radar technologies. The present PhD thesis is a study in the field of strongly correlated systems where coupled properties arise from the interplay of charge and spin degrees of freedom over lattice topologies enabling competing magnetic interactions and therefore emergence of coupling of electric and magnetic order. Non-perovskite, two-dimensional (2D) Na-Mn-O oxides are revisited in scope of this in both polycrystalline and large single crystal forms. Among Na-deficient polymorphs, hexagonal α-Na0.7MnO2 (single crystals) has been investigated for the first time as a playground of competing interactions due to mixed Mnvalence (Mn4+ / Mn3+), fostered by Na vacancies in the structure. The competition of FM (Mn3+-Mn4+) and AFM (Mn3+ -Mn3+) interactions is believed to be the origin of the magnetic instability leading to a glassy ground state leaving also their footprint in the dielectric permittivity measurements. Competing FM and AFΜ interactions are also investigated as the origin of the anisotropic magnetic properties witnessed in a-NaxMnO2 (x= 0.96) single crystals. Neutron single crystal experiments show a well-established AFM long range order which vanishes above 26 K whilea coexistent canted antiferromagnetic state persists up to 45 K. In both alpha powders and aNa0.96MnO2 single crystals, the dielectric permittivity suggests the onset of the commensuratemagnetic long range order (T~ 45 K) which in the case of the powders allows a magnetocapacitance effect. Compositional modulations in β-NaMnO2, which are depicted as an intergrowth of α- and βlike oxygen coordinations, are found to trigger a proper-screw magnetic ground state which evolves into collinear commensurate AFM state. Features in the dielectric permittivity coincide with the onset of the commensurate AFM order giving away also the contribution of the α- structural domains. Further understanding of the mechanisms that dictate the relief of frustrated interactions and establishment of magnetic order together with the role of structural complexity in the form of domains or domain-walls is a direction that warrants further exploration as it will help us to resolve whether other coupled electron degrees of freedom are likely to be generated in this family of oxides.


1990 ◽  
Vol 04 (18) ◽  
pp. 1143-1151 ◽  
Author(s):  
D.M. GAITONDE ◽  
SUMATHI RAO

We consider a model of anyons—fermions coupled to a statistical gauge field—also coupled to phonons via the usual electron-phonon coupling. We study the phonon response when the system is in the superconducting anyon ground state and show the existence of peaks in the phonon spectral function whose frequencies shift with carrier concentration. We suggest that when the electron-phonon coupling is the dominant effect, there could arise a new type of BCS ground state with a spontaneously generated P and T violating supercurrent, that expels the statistical magnetic field.


1998 ◽  
Vol 12 (28) ◽  
pp. 1167-1174
Author(s):  
W. Z. Wang ◽  
K. L. Yao ◽  
H. Q. Lin

Two kinds of alignments of two neighboring π-conjugated organic ferromagnetic chains are studied by considering the itinerary of electrons, electron–phonon coupling, the Hubbard repulsion and the interchain coupling. It is shown that the out-of-phase alignment is a more stable ferromagnetic structure than in-phase alignment. For out-of-phase alignment, there is a structure transition at a critical interchain coupling, at which the dimerizations of two chains have the same size and reverse sign. Interchain coupling results in transfer of spin density between each main chain and the side radicals.


1993 ◽  
Vol 71 (11-12) ◽  
pp. 493-500
Author(s):  
Y. Lépine ◽  
O. Schönborn

The ground-state energy of a bound polaron in a narrow-band polar crystal (such as a metal oxide) is studied using variational wave functions. We use a Fröhlich-type Hamiltonian on which the effective mass approximation has not been effected and in which a Debye cutoff is made on the phonon wave vectors. The wave functions that are used are general enough to allow the existence of a band state and of a self-trapped state and are reliable in the nonadiabatic limit. We find that three ground states are possible for this system. First, for small electron–phonon coupling, moderate bandwidth, and shallow impurities, the usual effective-mass hydrogenic ground state is found. For a narrow bandwidth and a deep defect, a collapsed state is predicted in which the polaron coincides with the position of the defect. Finally, for moderate electron–phonon coupling, narrow bandwidth, and a very weak defect, a self-trapped polaron in a hydrogenic state is predicted. Our conclusions are presented as asymptotic expansions and as phase diagrams indicating the values of the parameters for which each phase can be found.


2013 ◽  
Vol 27 (08) ◽  
pp. 1350050
Author(s):  
JUNHUA HOU ◽  
XIAOMING DONG ◽  
XIAOFENG DUAN

Self-trapping transition of the acoustic polaron in slab is researched by calculating the polaron ground state energy and the first derivative of the ground state energy with respect to the electron–phonon coupling. It is indicated that the possibility of self-trapping transition for acoustic polaron in slab fall in between 3D and 2D systems. The electron may be self-trapped in slab systems of GaN , AlN and alkali halides, if the slab systems are thinner than one over ten of the length unit ℏ/mc.


1980 ◽  
Vol 21 (4) ◽  
pp. 1432-1444 ◽  
Author(s):  
Talat S. Rahman ◽  
D. L. Mills

1987 ◽  
Vol 02 (01) ◽  
pp. 265-272 ◽  
Author(s):  
A. O. BARUT ◽  
R. RACZKA

The problem of long range forces between hadrons is re-examined. Without color degrees of freedom the confining potentials lead to unacceptably large van der Waals forces. The color degrees of freedom are investigated in the case of mesons taking into account both of the color singlet states of four constituents. The consistency of the color scheme requires the existence of four-quark bound states as well as two separate mesons and puts still restrictions on the confining potentials.


2011 ◽  
Vol 25 (01) ◽  
pp. 21-30
Author(s):  
WEI XIAO ◽  
JING-LIN XIAO

The Hamiltonian of a quantum rod with an ellipsoidal boundary is given after a coordinate transformation, which changes the ellipsoidal boundary into a spherical one. We then study the vibrational frequency and the ground state binding energy of the weak-coupling impurity bound magnetopolaron in it. The effects of the aspect ratio of the ellipsoid, the transverse effective confinement lengths, the electron-phonon coupling strength, the magnetic field cyclotron frequency and the Coulomb bound potential are taken into consideration by using linear combination operator method. It is found that the vibrational frequency and the ground state binding energy will increase with increasing Coulomb bound potential and the cyclotron frequency. They are decreasing functions of the aspect ratio of the ellipsoid and the transverse effective confinement lengths, whereas the ground state binding energy is an increasing function of the electron-phonon coupling strength.


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