Ahmet Nihat Berker

Berker, Ahmet Nihat

Name Variants

Nihat Berker A.
BERKER, Ahmet Nihat
Ahmet Nihat, Berker
Berker,Ahmet Nihat
Berker, AHMET NIHAT
A. Berker
B.,Ahmet Nihat
Berker,A.N.
Berker, A.
Berker, Ahmet Nihat
Berker N.
Ahmet Nihat Berker
Ahmet Nihat BERKER
Berker, A. N.
B., Ahmet Nihat
AHMET NIHAT BERKER
A. N. Berker
BERKER, AHMET NIHAT
Berker A.
Berker, A. Nihat

Job Title

Prof. Dr.

Email Address

nihatberker@khas.edu.tr

Main Affiliation

Electrical-Electronics Engineering

Sustainable Development Goals Report Points

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Scholarly Output

27

Articles

27

Citation Count

0

Supervised Theses

0 Scholarly Output Search Results

Now showing 1 - 10 of 27

Citation - WoS: 2
Citation - Scopus: 2
Driven and Non-Driven Surface Chaos in Spin-Glass Sponges
(Pergamon-elsevier Science Ltd, 2023) Pektas, Yigit Ertac; Berker, Ahmet Nihat; Artun, E. Can; Berker, A. Nihat; Electrical-Electronics Engineering
A spin-glass system with a smooth or fractal outer surface is studied by renormalization-group theory, in bulk spatial dimension d = 3. Independently varying the surface and bulk random-interaction strengths, phase diagrams are calculated. The smooth surface does not have spin-glass ordering in the absence of bulk spin-glass ordering and always has spin-glass ordering when the bulk is spin-glass ordered. With fractal (d > 2) surfaces, a sponge is obtained and has surface spin-glass ordering also in the absence of bulk spin-glass ordering. The phase diagram has the only-surface-spin-glass ordered phase, the bulk and surface spin-glass ordered phase, and the disordered phase, and a special multicritical point where these three phases meet. All spin-glass phases have distinct chaotic renormalization-group trajectories, with distinct Lyapunov and runaway exponents which we have calculated.
Citation - WoS: 1
Citation - Scopus: 1
Global Ashkin-Teller Phase Diagrams in Two and Three Dimensions: Multicritical Bifurcation Versus Double Tricriticality-Endpoint
(Elsevier, 2023) Kecoglu, Ibrahim; Berker, Ahmet Nihat; Berker, A. Nihat; Electrical-Electronics Engineering
The global phase diagrams of the Ashkin-Teller model are calculated in d = 2 and 3 by renormalization-group theory that is exact on the hierarchical lattice and approximate on the recently improved Migdal-Kadanoff procedure. Three different ordered phases occur in the dimensionally distinct phase diagrams that reflect three-fold order-parameter permutation symmetry, a closed symmetry line, and a quasi-disorder line. First- and second-order phase boundaries are obtained. In d = 2, second-order phase transitions meeting at a bifurcation point are seen. In d = 3, first- and second-order phase transitions are separated by tricritical and critical endpoints.
Citation - WoS: 11
Citation - Scopus: 11
Phase Transitions Between Different Spin-Glass Phases and Between Different Chaoses in Quenched Random Chiral Systems
(Amer Physical Soc., 2017) Çağlar, Tolga; Berker, Ahmet Nihat; Berker, A. Nihat; Electrical-Electronics Engineering
The left-right chiral and ferromagnetic-antiferromagnetic double-spin-glass clock model with the crucially even number of states q = 4 and in three dimensions d = 3 has been studied by renormalization-group theory. We find for the first time to our knowledge four spin-glass phases including conventional chiral and quadrupolar spin-glass phases and phase transitions between spin-glass phases. The chaoses in the different spin-glass phases and in the phase transitions of the spin-glass phases with the other spin-glass phases with the non-spin-glass ordered phases and with the disordered phase are determined and quantified by Lyapunov exponents. It is seen that the chiral spin-glass phase is the most chaotic spin-glass phase. The calculated phase diagram is also otherwise very rich including regular and temperature-inverted devil's staircases and reentrances.
Citation - WoS: 2
Citation - Scopus: 2
Renormalization-Group Theory of the Heisenberg Model in D Dimensions
(Elsevier, 2022) Tunca, Egemen; Berker, Ahmet Nihat; Berker, A. Nihat; Electrical-Electronics Engineering
The classical Heisenberg model has been solved in spatial d dimensions, exactly in d = 1 and by the Migdal-Kadanoff approximation in d > 1, by using a Fourier-Legendre expansion. The phase transition temperatures, the energy densities, and the specific heats are calculated in arbitrary dimension d. Fisher's exact result is recovered in d = 1. The absence of an ordered phase, conventional or algebraic (in contrast to the XY model yielding an algebraically ordered phase) is recovered in d = 2. A conventionally ordered phase occurs at d > 2. This method opens the way to complex-system calculations with Heisenberg local degrees of freedom.(c) 2022 Elsevier B.V. All rights reserved.
Citation - WoS: 0
Citation - Scopus: 0
Axial, planar-diagonal, body-diagonal fields on the cubic-spin spin glass in d=3: A plethora of ordered phases under finite fields
(Amer Physical Soc, 2024) Berker, Ahmet Nihat; Sarman, Deniz; Berker, A. Nihat; Electrical-Electronics Engineering
A nematic phase, previously seen in the d = 3 classical Heisenberg spin-glass system, occurs in the n-component cubic-spin spin-glass system, between the low-temperature spin-glass phase and the hightemperature disordered phase, for number of spin components n >= 3, in spatial dimension d = 3, thus constituting a liquid-crystal phase in a dirty (quenched-disordered) magnet. Furthermore, under application of a variety of uniform magnetic fields, a veritable plethora of phases is found. Under uniform magnetic fields, 17 different phases and two spin-glass phase diagram topologies (meaning the occurrences and relative positions of the many phases), qualitatively different from the conventional spin-glass phase diagram topology, are seen. The chaotic rescaling behaviors and their Lyapunov exponents are calculated in each of these spin-glass phase diagram topologies. These results are obtained from renormalization-group calculations that are exact on the d = 3 hierarchical lattice and, equivalently, approximate on the cubic spatial lattice. Axial, planar-diagonal, or body-diagonal finite-strength uniform fields are applied to n = 2 and 3 component cubic-spin spin-glass systems in d=3.
Citation - WoS: 7
Citation - Scopus: 7
Lower Critical Dimension of the Random-Field Xy Model and the Zero-Temperature Critical Line
(Amer Physical Soc, 2022) Akin, Kutay; Berker, Ahmet Nihat; Berker, A. Nihat; Electrical-Electronics Engineering
The random-field XY model is studied in spatial dimensions d = 3 and 4, and in between, as the limit q -> infinity of the q-state clock models, by the exact renormalization-group solution of the hierarchical lattice or, equivalently, the Migdal-Kadanoff approximation to the hypercubic lattices. The lower critical dimension is determined between 3.81 < d(c) < 4. When the random field is scaled with q, a line segment of zero-temperature criticality is found in d = 3. When the random field is scaled with q(2), a universal phase diagram is found at intermediate temperatures in d = 3.
Citation - WoS: 4
Citation - Scopus: 4
Phase Transitions of the Variety of Random-Field Potts Models
(Elsevier, 2021) Turkoglu, Alpar; Berker, Ahmet Nihat; Berker, A. Nihat; Electrical-Electronics Engineering
The phase transitions of random-field q-state Potts models in d = 3 dimensions are studied by renormalization-group theory by exact solution of a hierarchical lattice and, equivalently, approximate Migdal-Kadanoff solutions of a cubic lattice. The recursion, under rescaling, of coupled random-field and random-bond (induced under rescaling by random fields) coupled probability distributions is followed to obtain phase diagrams. Unlike the Ising model (q = 2), several types of random fields can be defined for q >= 3 Potts models, including random-axis favored, random-axis disfavored, random-axis randomly favored or disfavored cases, all of which are studied. Quantitatively very similar phase diagrams are obtained, for a given q for the three types of field randomness, with the low-temperature ordered phase persisting, increasingly as temperature is lowered, up to random-field threshold in d = 3, which is calculated for all temperatures below the zero-field critical temperature. Phase diagrams thus obtained are compared as a function of q. The ordered phase in the low-q models reaches higher temperatures, while in the high-q models it reaches higher random fields. This renormalization-group calculation result is physically explained. (c) 2021 Elsevier B.V. All rights reserved.
Citation - WoS: 0
Citation - Scopus: 0
Reentrant Ferromagnetic Ordering of the Random-Field Heisenberg Model in d > 2 Dimensions: Fourier-Legendre Renormalization-Group Theory
(Amer Physical Soc, 2024) Tuerkoglu, Alpar; Berker, Ahmet Nihat; Berker, A. Nihat; Electrical-Electronics Engineering
The random-magnetic-field classical Heisenberg model is solved in spatial dimensions d >= 2 using the recently developed Fourier-Legendre renormalization-group theory for 47r steradians continuously orientable spins, with renormalization-group flows of 12 500 variables. The random-magnetic-field Heisenberg model is exactly solved in 10 hierarchical models, for d = 2, 2.26, 2.46, 2.58, 2.63, 2.77, 2.89, 3. For nonzero random fields, ferromagnetic order is seen ford > 2. This ordering, at d = 2.46, 2.58, 2.63, 2.77, 2.89, 3, shows reentrance as a function of temperature.
Citation - WoS: 10
Citation - Scopus: 10
Devil's Staircase Continuum in the Chiral Clock Spin Glass With Competing Ferromagnetic-Antiferromagnetic and Left-Right Chiral Interactions
(Amer Physical Soc., 2017) Caglar, Tolga; Berker, Ahmet Nihat; Berker, A. Nihat; Electrical-Electronics Engineering
The chiral clock spin-glass model with q = 5 states with both competing ferromagnetic-antiferromagnetic and left-right chiral frustrations is studied in d = 3 spatial dimensions by renormalization-group theory. The global phase diagram is calculated in temperature antiferromagnetic bond concentration p random chirality strength and right-chirality concentration c. The system has a ferromagnetic phase a multitude of different chiral phases a chiral spin-glass phase and a critical (algebraically) ordered phase. The ferromagnetic and chiral phases accumulate at the disordered phase boundary and form a spectrum of devil's staircases where different ordered phases characteristically intercede at all scales of phase-diagram space. Shallow and deep reentrances of the disordered phase bordered by fragments of regular and temperature-inverted devil's staircases are seen. The extremely rich phase diagrams are presented as continuously and qualitatively changing videos.
Citation - WoS: 0
Citation - Scopus: 0
Metastable Potts droplets
(AMER PHYSICAL SOC, 2021-03) Berker, Ahmet Nihat; Berker, A. Nihat; Electrical-Electronics Engineering
The existence and limits of metastable droplets have been calculated using finite-system renormalization-group theory, for q-state Potts models in spatial dimension d = 3. The dependence of the droplet critical sizes on magnetic field, temperature, and number of Potts states q has been calculated. The same method has also been used for the calculation of hysteresis loops across first-order phase transitions in these systems. The hysteresis loop sizes and shapes have been deduced as a function of magnetic field, temperature, and number of Potts states q. The uneven appearance of asymmetry in the hysteresis loop branches has been noted. The method can be extended to criticality and phase transitions in metastable phases, such as in surface-adsorbed systems and water.